<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:01a0cb90-d0f2-7e19-b102-2cb501edcbca,g:8395393c-5e1d-497e-a1c7-f0fc68563741-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.pulmonx.com/role/CondensedConsolidatedStatementsofStockholdersEquity" xlink:type="simple" xlink:href="lung-20260930.xsd#CondensedConsolidatedStatementsofStockholdersEquity"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/CondensedConsolidatedStatementsofStockholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_01a0cb90-d1a6-7217-997d-bbe0baa7a3b7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_01a0cb90-d1a6-7cda-9732-786e816cab30" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_01a0cb90-d1a6-7217-997d-bbe0baa7a3b7" xlink:to="loc_us-gaap_StatementTable_01a0cb90-d1a6-7cda-9732-786e816cab30" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_01a0cb90-d1a6-70b0-96fa-75cbec767028" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_01a0cb90-d1a6-7cda-9732-786e816cab30" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_01a0cb90-d1a6-70b0-96fa-75cbec767028" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a0cb90-d1a6-70b0-96fa-75cbec767028_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a0cb90-d1a6-70b0-96fa-75cbec767028" xlink:to="loc_us-gaap_EquityComponentDomain_01a0cb90-d1a6-70b0-96fa-75cbec767028_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_01a0cb90-d1a6-76ad-9f5f-2cd62eec6660" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_01a0cb90-d1a6-70b0-96fa-75cbec767028" xlink:to="loc_us-gaap_EquityComponentDomain_01a0cb90-d1a6-76ad-9f5f-2cd62eec6660" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_01a0cb90-d1a6-7d3e-836a-92b47c4f1933" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a0cb90-d1a6-76ad-9f5f-2cd62eec6660" xlink:to="loc_us-gaap_CommonStockMember_01a0cb90-d1a6-7d3e-836a-92b47c4f1933" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_01a0cb90-d1a6-77bf-96bf-f3ffe6ff6b38" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a0cb90-d1a6-76ad-9f5f-2cd62eec6660" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_01a0cb90-d1a6-77bf-96bf-f3ffe6ff6b38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_01a0cb90-d1a6-756f-b9ef-00c9ad4d1dbb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a0cb90-d1a6-76ad-9f5f-2cd62eec6660" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_01a0cb90-d1a6-756f-b9ef-00c9ad4d1dbb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_01a0cb90-d1a6-73a4-90e2-d9d3ba45709b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_01a0cb90-d1a6-76ad-9f5f-2cd62eec6660" xlink:to="loc_us-gaap_RetainedEarningsMember_01a0cb90-d1a6-73a4-90e2-d9d3ba45709b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_01a0cb90-d1a6-7217-997d-bbe0baa7a3b7" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_01a0cb90-d1a6-76e6-97fe-2a05c3ebedea" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_01a0cb90-d1a6-76e6-97fe-2a05c3ebedea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_01a0cb90-d1a6-756b-bb88-7d459bbd08c5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_StockholdersEquity_01a0cb90-d1a6-756b-bb88-7d459bbd08c5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_01a0cb90-d1a6-7312-8ae9-08d9445314c7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesRestrictedStockAwardNetOfForfeitures_01a0cb90-d1a6-7312-8ae9-08d9445314c7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures_01a0cb90-d1a6-766d-89b7-a0d5ccfc24fd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueRestrictedStockAwardNetOfForfeitures_01a0cb90-d1a6-766d-89b7-a0d5ccfc24fd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_01a0cb90-d1a6-7dd8-978e-8119f8d36d1d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_01a0cb90-d1a6-7dd8-978e-8119f8d36d1d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_01a0cb90-d1a6-7c95-b259-6ec39d11925e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueStockOptionsExercised_01a0cb90-d1a6-7c95-b259-6ec39d11925e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_01a0cb90-d1a6-7469-8c46-dd71ec86ffba" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_01a0cb90-d1a6-7469-8c46-dd71ec86ffba" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_01a0cb90-d1a6-793f-9a56-bbae3e35a623" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueEmployeeStockPurchasePlan_01a0cb90-d1a6-793f-9a56-bbae3e35a623" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued_01a0cb90-d1a6-72ec-8a6c-576b2e135ec7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalWarrantIssued_01a0cb90-d1a6-72ec-8a6c-576b2e135ec7" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_01a0cb90-d1a6-73e2-a7a1-ab6cb005e330" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_01a0cb90-d1a6-73e2-a7a1-ab6cb005e330" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_01a0cb90-d1a6-7823-8e6c-8ad6af64ab8b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_01a0cb90-d1a6-7823-8e6c-8ad6af64ab8b" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax_01a0cb90-d1a6-74c1-9f3a-44027b9c4a59" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentNetOfTax_01a0cb90-d1a6-74c1-9f3a-44027b9c4a59" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_01a0cb90-d1a6-7980-91ec-a8c055bdc12f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_01a0cb90-d1a6-7e56-802c-219461b9c46d" xlink:to="loc_us-gaap_NetIncomeLoss_01a0cb90-d1a6-7980-91ec-a8c055bdc12f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_01a0cb90-d1a6-7ee3-91fb-65d18bca2d5a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_01a0cb90-d1a6-7819-af0e-5ba08dcd8165" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/FairValueMeasurementsScheduleofFairValueofAssetsandLiabilitiesDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#FairValueMeasurementsScheduleofFairValueofAssetsandLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/FairValueMeasurementsScheduleofFairValueofAssetsandLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0cb90-d1a6-7343-8db0-7782ae947fc3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a0cb90-d1a6-78cb-b690-cd1ba92f8f3c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0cb90-d1a6-7343-8db0-7782ae947fc3" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a0cb90-d1a6-78cb-b690-cd1ba92f8f3c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a0cb90-d1a6-7524-a006-1deb74deb933" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a0cb90-d1a6-78cb-b690-cd1ba92f8f3c" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a0cb90-d1a6-7524-a006-1deb74deb933" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a0cb90-d1a6-7524-a006-1deb74deb933_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a0cb90-d1a6-7524-a006-1deb74deb933" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a0cb90-d1a6-7524-a006-1deb74deb933_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a0cb90-d1a6-77ef-993b-e80f60ad9861" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a0cb90-d1a6-7524-a006-1deb74deb933" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a0cb90-d1a6-77ef-993b-e80f60ad9861" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_01a0cb90-d1a6-7281-acc3-6ca328280d4f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a0cb90-d1a6-77ef-993b-e80f60ad9861" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_01a0cb90-d1a6-7281-acc3-6ca328280d4f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a0cb90-d1a6-7fda-811e-6edc06b435da" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a0cb90-d1a6-78cb-b690-cd1ba92f8f3c" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a0cb90-d1a6-7fda-811e-6edc06b435da" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0cb90-d1a6-7fda-811e-6edc06b435da_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a0cb90-d1a6-7fda-811e-6edc06b435da" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0cb90-d1a6-7fda-811e-6edc06b435da_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0cb90-d1a6-731c-87fd-683f82e1f11d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_01a0cb90-d1a6-7fda-811e-6edc06b435da" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0cb90-d1a6-731c-87fd-683f82e1f11d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_01a0cb90-d1a6-7097-a3d6-4dd5bcbba6fc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0cb90-d1a6-731c-87fd-683f82e1f11d" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_01a0cb90-d1a6-7097-a3d6-4dd5bcbba6fc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_01a0cb90-d1a6-76f0-a5b1-10af7cdd2773" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0cb90-d1a6-731c-87fd-683f82e1f11d" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_01a0cb90-d1a6-76f0-a5b1-10af7cdd2773" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_01a0cb90-d1a6-74b6-ae19-6bade95037d3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_01a0cb90-d1a6-731c-87fd-683f82e1f11d" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_01a0cb90-d1a6-74b6-ae19-6bade95037d3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_01a0cb90-d1a6-74c7-a567-2f878c8d8e83" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a0cb90-d1a6-78cb-b690-cd1ba92f8f3c" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_01a0cb90-d1a6-74c7-a567-2f878c8d8e83" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a0cb90-d1a6-74c7-a567-2f878c8d8e83_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_01a0cb90-d1a6-74c7-a567-2f878c8d8e83" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a0cb90-d1a6-74c7-a567-2f878c8d8e83_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a0cb90-d1a6-7e61-9852-e87e6c7a6353" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_01a0cb90-d1a6-74c7-a567-2f878c8d8e83" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a0cb90-d1a6-7e61-9852-e87e6c7a6353" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_01a0cb90-d1a6-74d3-8223-7e7a25474f59" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_01a0cb90-d1a6-7e61-9852-e87e6c7a6353" xlink:to="loc_us-gaap_MoneyMarketFundsMember_01a0cb90-d1a6-74d3-8223-7e7a25474f59" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_01a0cb90-d1a6-70c0-93a4-2793dcb47642" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0cb90-d1a6-7343-8db0-7782ae947fc3" xlink:to="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_01a0cb90-d1a6-70c0-93a4-2793dcb47642" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_01a0cb90-d1a6-737e-9041-dcaa9ab06311" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_01a0cb90-d1a6-70c0-93a4-2793dcb47642" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_01a0cb90-d1a6-737e-9041-dcaa9ab06311" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_01a0cb90-d1a6-754d-bf31-f23d2f4d6167" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialAssetsBalanceSheetGroupingsAbstract_01a0cb90-d1a6-70c0-93a4-2793dcb47642" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_01a0cb90-d1a6-754d-bf31-f23d2f4d6167" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_01a0cb90-d1a6-7e5d-92f9-68438ff40c9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0cb90-d1a6-7343-8db0-7782ae947fc3" xlink:to="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_01a0cb90-d1a6-7e5d-92f9-68438ff40c9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_01a0cb90-d1a6-7bc2-89ad-ae9b73e9f20b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_01a0cb90-d1a6-7e5d-92f9-68438ff40c9d" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_01a0cb90-d1a6-7bc2-89ad-ae9b73e9f20b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_01a0cb90-d1a6-7737-b6af-ea7f1774cec9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FinancialInstrumentsFinancialLiabilitiesBalanceSheetGroupingsAbstract_01a0cb90-d1a6-7e5d-92f9-68438ff40c9d" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_01a0cb90-d1a6-7737-b6af-ea7f1774cec9" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#FairValueMeasurementsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/FairValueMeasurementsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeLineItems_01a0cb90-d1a6-762d-a807-a468e1611613" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DerivativeTable_01a0cb90-d1a6-7b7f-9e40-4e58befd9c5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DerivativeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_01a0cb90-d1a6-762d-a807-a468e1611613" xlink:to="loc_us-gaap_DerivativeTable_01a0cb90-d1a6-7b7f-9e40-4e58befd9c5b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a0cb90-d1a6-775d-b53b-38ac44c22f66" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_01a0cb90-d1a6-7b7f-9e40-4e58befd9c5b" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a0cb90-d1a6-775d-b53b-38ac44c22f66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a0cb90-d1a6-775d-b53b-38ac44c22f66_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a0cb90-d1a6-775d-b53b-38ac44c22f66" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a0cb90-d1a6-775d-b53b-38ac44c22f66_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a0cb90-d1a6-74ef-8a71-d0e01a77e98b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_01a0cb90-d1a6-775d-b53b-38ac44c22f66" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a0cb90-d1a6-74ef-8a71-d0e01a77e98b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_01a0cb90-d1a6-731b-8b6c-1b98f66d5d85" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a0cb90-d1a6-74ef-8a71-d0e01a77e98b" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_01a0cb90-d1a6-731b-8b6c-1b98f66d5d85" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsNonrecurringMember_01a0cb90-d1a6-71f7-98e3-aca321f6d55a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueMeasurementsNonrecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_01a0cb90-d1a6-74ef-8a71-d0e01a77e98b" xlink:to="loc_us-gaap_FairValueMeasurementsNonrecurringMember_01a0cb90-d1a6-71f7-98e3-aca321f6d55a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialLiabilitiesFairValueDisclosure_01a0cb90-d1a6-717b-afcb-05754c2db869" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FinancialLiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_01a0cb90-d1a6-762d-a807-a468e1611613" xlink:to="loc_us-gaap_FinancialLiabilitiesFairValueDisclosure_01a0cb90-d1a6-717b-afcb-05754c2db869" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_01a0cb90-d1a6-7f54-8135-02edcfb6e0df" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_01a0cb90-d1a6-762d-a807-a468e1611613" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_01a0cb90-d1a6-7f54-8135-02edcfb6e0df" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/FairValueMeasurementsValuationAssumptionsDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#FairValueMeasurementsValuationAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/FairValueMeasurementsValuationAssumptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0cb90-d1a6-7392-b96e-73a923186fc6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a0cb90-d1a6-7540-82aa-17c3dcc0887b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0cb90-d1a6-7392-b96e-73a923186fc6" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a0cb90-d1a6-7540-82aa-17c3dcc0887b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_01a0cb90-d1a6-7b69-9136-46545989409e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_01a0cb90-d1a6-7540-82aa-17c3dcc0887b" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_01a0cb90-d1a6-7b69-9136-46545989409e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d1a6-7b69-9136-46545989409e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_01a0cb90-d1a6-7b69-9136-46545989409e" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d1a6-7b69-9136-46545989409e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d1a6-74d6-86a7-f7673ff6fa2c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_01a0cb90-d1a6-7b69-9136-46545989409e" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d1a6-74d6-86a7-f7673ff6fa2c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputSharePriceMember_01a0cb90-d1a6-7aec-97e5-769800cec2d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputSharePriceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d1a6-74d6-86a7-f7673ff6fa2c" xlink:to="loc_us-gaap_MeasurementInputSharePriceMember_01a0cb90-d1a6-7aec-97e5-769800cec2d4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedTermMember_01a0cb90-d1a6-737f-b3a3-7f9dec84129c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputExpectedTermMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d1a6-74d6-86a7-f7673ff6fa2c" xlink:to="loc_us-gaap_MeasurementInputExpectedTermMember_01a0cb90-d1a6-737f-b3a3-7f9dec84129c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputPriceVolatilityMember_01a0cb90-d1a6-7e45-b3dd-1ae2c7500eb3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputPriceVolatilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d1a6-74d6-86a7-f7673ff6fa2c" xlink:to="loc_us-gaap_MeasurementInputPriceVolatilityMember_01a0cb90-d1a6-7e45-b3dd-1ae2c7500eb3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_01a0cb90-d1a6-7e6c-ad19-a5adc0efbefd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d1a6-74d6-86a7-f7673ff6fa2c" xlink:to="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_01a0cb90-d1a6-7e6c-ad19-a5adc0efbefd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedDividendRateMember_01a0cb90-d1a6-7a30-8c36-cecedf5e9466" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputExpectedDividendRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d1a6-74d6-86a7-f7673ff6fa2c" xlink:to="loc_us-gaap_MeasurementInputExpectedDividendRateMember_01a0cb90-d1a6-7a30-8c36-cecedf5e9466" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_01a0cb90-d1a6-733d-a0a4-0b1bf53f320e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstandingMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_01a0cb90-d1a6-7392-b96e-73a923186fc6" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_01a0cb90-d1a6-733d-a0a4-0b1bf53f320e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/BalanceSheetComponentsCapitalizedImplementationCostsofaHostingArrangementDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#BalanceSheetComponentsCapitalizedImplementationCostsofaHostingArrangementDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/BalanceSheetComponentsCapitalizedImplementationCostsofaHostingArrangementDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-7639-a087-60855d3e030a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0cb90-d253-7a00-8fb9-05cddb2ff2cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-7639-a087-60855d3e030a" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0cb90-d253-7a00-8fb9-05cddb2ff2cd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-74ea-9be3-0674e2201fc3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0cb90-d253-7a00-8fb9-05cddb2ff2cd" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-74ea-9be3-0674e2201fc3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-74ea-9be3-0674e2201fc3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-74ea-9be3-0674e2201fc3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-74ea-9be3-0674e2201fc3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-7fa6-886e-570542cbc241" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-74ea-9be3-0674e2201fc3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-7fa6-886e-570542cbc241" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_01a0cb90-d253-7496-8ce8-779723c4211f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-7fa6-886e-570542cbc241" xlink:to="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_01a0cb90-d253-7496-8ce8-779723c4211f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_01a0cb90-d253-7bb4-9931-36943e207d41" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0cb90-d253-7a00-8fb9-05cddb2ff2cd" xlink:to="loc_srt_RangeAxis_01a0cb90-d253-7bb4-9931-36943e207d41" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a0cb90-d253-7bb4-9931-36943e207d41_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_01a0cb90-d253-7bb4-9931-36943e207d41" xlink:to="loc_srt_RangeMember_01a0cb90-d253-7bb4-9931-36943e207d41_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a0cb90-d253-7b85-adfb-dfbe8480d043" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_01a0cb90-d253-7bb4-9931-36943e207d41" xlink:to="loc_srt_RangeMember_01a0cb90-d253-7b85-adfb-dfbe8480d043" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_01a0cb90-d253-77a7-98d5-567b736a0d35" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a0cb90-d253-7b85-adfb-dfbe8480d043" xlink:to="loc_srt_MinimumMember_01a0cb90-d253-77a7-98d5-567b736a0d35" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_01a0cb90-d253-70a5-88ef-0d80419298e7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a0cb90-d253-7b85-adfb-dfbe8480d043" xlink:to="loc_srt_MaximumMember_01a0cb90-d253-70a5-88ef-0d80419298e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfFinancialPositionLocationBalanceAxis_01a0cb90-d253-730e-adee-3d7d252ed17f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfFinancialPositionLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0cb90-d253-7a00-8fb9-05cddb2ff2cd" xlink:to="loc_us-gaap_StatementOfFinancialPositionLocationBalanceAxis_01a0cb90-d253-730e-adee-3d7d252ed17f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HostingArrangementServiceContractImplementationCostCapitalizedAfterAccumulatedAmortization_01a0cb90-d253-7b7c-9a8e-ac3314b6ae8c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HostingArrangementServiceContractImplementationCostCapitalizedAfterAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-7639-a087-60855d3e030a" xlink:to="loc_us-gaap_HostingArrangementServiceContractImplementationCostCapitalizedAfterAccumulatedAmortization_01a0cb90-d253-7b7c-9a8e-ac3314b6ae8c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_HostingArrangementServiceContractImplementationCostCapitalizedAmortizationPeriod_01a0cb90-d253-7133-a868-35ff9e88953f" xlink:href="lung-20260930.xsd#lung_HostingArrangementServiceContractImplementationCostCapitalizedAmortizationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-7639-a087-60855d3e030a" xlink:to="loc_lung_HostingArrangementServiceContractImplementationCostCapitalizedAmortizationPeriod_01a0cb90-d253-7133-a868-35ff9e88953f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HostingArrangementServiceContractImplementationCostExpenseAmortization_01a0cb90-d253-76f2-bb1a-ba3cdd2ff24f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HostingArrangementServiceContractImplementationCostExpenseAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-7639-a087-60855d3e030a" xlink:to="loc_us-gaap_HostingArrangementServiceContractImplementationCostExpenseAmortization_01a0cb90-d253-76f2-bb1a-ba3cdd2ff24f" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/BalanceSheetComponentsCapitalizedImplementationCostsofaHostingArrangementDetails_1" xlink:type="simple" xlink:href="lung-20260930.xsd#BalanceSheetComponentsCapitalizedImplementationCostsofaHostingArrangementDetails_1"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/BalanceSheetComponentsCapitalizedImplementationCostsofaHostingArrangementDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-7639-a087-60855d3e030a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0cb90-d253-7a00-8fb9-05cddb2ff2cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-7639-a087-60855d3e030a" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0cb90-d253-7a00-8fb9-05cddb2ff2cd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-74ea-9be3-0674e2201fc3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0cb90-d253-7a00-8fb9-05cddb2ff2cd" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-74ea-9be3-0674e2201fc3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-74ea-9be3-0674e2201fc3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-74ea-9be3-0674e2201fc3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-74ea-9be3-0674e2201fc3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-7fa6-886e-570542cbc241" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-74ea-9be3-0674e2201fc3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-7fa6-886e-570542cbc241" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_01a0cb90-d253-7496-8ce8-779723c4211f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-7fa6-886e-570542cbc241" xlink:to="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_01a0cb90-d253-7496-8ce8-779723c4211f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_01a0cb90-d253-7bb4-9931-36943e207d41" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0cb90-d253-7a00-8fb9-05cddb2ff2cd" xlink:to="loc_srt_RangeAxis_01a0cb90-d253-7bb4-9931-36943e207d41" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a0cb90-d253-7bb4-9931-36943e207d41_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_01a0cb90-d253-7bb4-9931-36943e207d41" xlink:to="loc_srt_RangeMember_01a0cb90-d253-7bb4-9931-36943e207d41_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a0cb90-d253-7b85-adfb-dfbe8480d043" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_01a0cb90-d253-7bb4-9931-36943e207d41" xlink:to="loc_srt_RangeMember_01a0cb90-d253-7b85-adfb-dfbe8480d043" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_01a0cb90-d253-77a7-98d5-567b736a0d35" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a0cb90-d253-7b85-adfb-dfbe8480d043" xlink:to="loc_srt_MinimumMember_01a0cb90-d253-77a7-98d5-567b736a0d35" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_01a0cb90-d253-70a5-88ef-0d80419298e7" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a0cb90-d253-7b85-adfb-dfbe8480d043" xlink:to="loc_srt_MaximumMember_01a0cb90-d253-70a5-88ef-0d80419298e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HostingArrangementServiceContractImplementationCostCapitalizedAfterAccumulatedAmortization_01a0cb90-d253-7b7c-9a8e-ac3314b6ae8c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HostingArrangementServiceContractImplementationCostCapitalizedAfterAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-7639-a087-60855d3e030a" xlink:to="loc_us-gaap_HostingArrangementServiceContractImplementationCostCapitalizedAfterAccumulatedAmortization_01a0cb90-d253-7b7c-9a8e-ac3314b6ae8c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_HostingArrangementServiceContractImplementationCostCapitalizedAmortizationPeriod_01a0cb90-d253-7133-a868-35ff9e88953f" xlink:href="lung-20260930.xsd#lung_HostingArrangementServiceContractImplementationCostCapitalizedAmortizationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-7639-a087-60855d3e030a" xlink:to="loc_lung_HostingArrangementServiceContractImplementationCostCapitalizedAmortizationPeriod_01a0cb90-d253-7133-a868-35ff9e88953f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_HostingArrangementServiceContractImplementationCostExpenseAmortization_01a0cb90-d253-76f2-bb1a-ba3cdd2ff24f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_HostingArrangementServiceContractImplementationCostExpenseAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-7639-a087-60855d3e030a" xlink:to="loc_us-gaap_HostingArrangementServiceContractImplementationCostExpenseAmortization_01a0cb90-d253-76f2-bb1a-ba3cdd2ff24f" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/BalanceSheetComponentsScheduleofPropertyandEquipmentNetDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#BalanceSheetComponentsScheduleofPropertyandEquipmentNetDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/BalanceSheetComponentsScheduleofPropertyandEquipmentNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-79ff-8fef-5a48c7905ce9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0cb90-d253-761c-b38b-d6e8a0426155" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-79ff-8fef-5a48c7905ce9" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0cb90-d253-761c-b38b-d6e8a0426155" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-7ea2-b616-a1d6664b0fbc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_01a0cb90-d253-761c-b38b-d6e8a0426155" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-7ea2-b616-a1d6664b0fbc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-7ea2-b616-a1d6664b0fbc_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-7ea2-b616-a1d6664b0fbc" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-7ea2-b616-a1d6664b0fbc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-75d3-b391-dc3c15f94222" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-7ea2-b616-a1d6664b0fbc" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-75d3-b391-dc3c15f94222" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_01a0cb90-d253-712e-800b-48b270a49d59" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MachineryAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-75d3-b391-dc3c15f94222" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_01a0cb90-d253-712e-800b-48b270a49d59" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_01a0cb90-d253-72b1-a0cd-c5d8458a4e10" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-75d3-b391-dc3c15f94222" xlink:to="loc_us-gaap_ComputerEquipmentMember_01a0cb90-d253-72b1-a0cd-c5d8458a4e10" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_01a0cb90-d253-7427-a083-9fa72d0deba8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-75d3-b391-dc3c15f94222" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_01a0cb90-d253-7427-a083-9fa72d0deba8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdImprovementsMember_01a0cb90-d253-7dab-aedf-2049d85af033" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-75d3-b391-dc3c15f94222" xlink:to="loc_us-gaap_LeaseholdImprovementsMember_01a0cb90-d253-7dab-aedf-2049d85af033" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_01a0cb90-d253-72c2-acd8-5aa655e1c060" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-75d3-b391-dc3c15f94222" xlink:to="loc_us-gaap_ConstructionInProgressMember_01a0cb90-d253-72c2-acd8-5aa655e1c060" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_01a0cb90-d253-7d35-816d-cc76311967a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-79ff-8fef-5a48c7905ce9" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_01a0cb90-d253-7d35-816d-cc76311967a6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_01a0cb90-d253-78e8-a4af-8d48b5ac456b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-79ff-8fef-5a48c7905ce9" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_01a0cb90-d253-78e8-a4af-8d48b5ac456b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_01a0cb90-d253-7c9f-9e4e-69a6c6639b89" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_01a0cb90-d253-79ff-8fef-5a48c7905ce9" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_01a0cb90-d253-7c9f-9e4e-69a6c6639b89" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/BalanceSheetComponentsNarrativeDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#BalanceSheetComponentsNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/BalanceSheetComponentsNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_01a0cb90-d253-76fc-a047-407c81827632" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a0cb90-d253-7885-8ace-b89d52db5b97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a0cb90-d253-76fc-a047-407c81827632" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a0cb90-d253-7885-8ace-b89d52db5b97" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_01a0cb90-d253-704f-83f1-92cced480166" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_01a0cb90-d253-7885-8ace-b89d52db5b97" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_01a0cb90-d253-704f-83f1-92cced480166" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a0cb90-d253-704f-83f1-92cced480166_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_01a0cb90-d253-704f-83f1-92cced480166" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a0cb90-d253-704f-83f1-92cced480166_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a0cb90-d253-732d-bde9-6ae7638dfbb4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_01a0cb90-d253-704f-83f1-92cced480166" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a0cb90-d253-732d-bde9-6ae7638dfbb4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_EmphasysMedicalIncMember_01a0cb90-d253-7199-b9af-86a35ad1e58d" xlink:href="lung-20260930.xsd#lung_EmphasysMedicalIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_01a0cb90-d253-732d-bde9-6ae7638dfbb4" xlink:to="loc_lung_EmphasysMedicalIncMember_01a0cb90-d253-7199-b9af-86a35ad1e58d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_01a0cb90-d253-7d28-9e37-a3b4f48f4269" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a0cb90-d253-76fc-a047-407c81827632" xlink:to="loc_us-gaap_Depreciation_01a0cb90-d253-7d28-9e37-a3b4f48f4269" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_01a0cb90-d253-7276-92f3-bd2bfcdf0cc3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a0cb90-d253-76fc-a047-407c81827632" xlink:to="loc_us-gaap_Goodwill_01a0cb90-d253-7276-92f3-bd2bfcdf0cc3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_01a0cb90-d253-7a78-a0bf-6f65eac146ed" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillAcquiredDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a0cb90-d253-76fc-a047-407c81827632" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_01a0cb90-d253-7a78-a0bf-6f65eac146ed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit_01a0cb90-d253-7f81-8b66-ddb060a59cc9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a0cb90-d253-76fc-a047-407c81827632" xlink:to="loc_us-gaap_GoodwillWrittenOffRelatedToSaleOfBusinessUnit_01a0cb90-d253-7f81-8b66-ddb060a59cc9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillImpairmentLoss_01a0cb90-d253-73e3-8339-61ef2d2f56c0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GoodwillImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_01a0cb90-d253-76fc-a047-407c81827632" xlink:to="loc_us-gaap_GoodwillImpairmentLoss_01a0cb90-d253-73e3-8339-61ef2d2f56c0" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/LongTermDebtCIBCLoanNarrativeDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#LongTermDebtCIBCLoanNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/LongTermDebtCIBCLoanNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-7a66-b44b-850836e303a1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-7a66-b44b-850836e303a1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7514-b610-0e43e23195e5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-7a66-b44b-850836e303a1" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7514-b610-0e43e23195e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7514-b610-0e43e23195e5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7514-b610-0e43e23195e5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7514-b610-0e43e23195e5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-70da-8874-9f3d7d54fce6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7514-b610-0e43e23195e5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-70da-8874-9f3d7d54fce6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_CIBCAgreementMember_01a0cb90-d253-7fc8-bb8e-3b889a3ee307" xlink:href="lung-20260930.xsd#lung_CIBCAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-70da-8874-9f3d7d54fce6" xlink:to="loc_lung_CIBCAgreementMember_01a0cb90-d253-7fc8-bb8e-3b889a3ee307" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_CIBCAgreementTrancheAMember_01a0cb90-d253-7bbc-a600-dd96c12443f7" xlink:href="lung-20260930.xsd#lung_CIBCAgreementTrancheAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-70da-8874-9f3d7d54fce6" xlink:to="loc_lung_CIBCAgreementTrancheAMember_01a0cb90-d253-7bbc-a600-dd96c12443f7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_CIBCAgreementTrancheBMember_01a0cb90-d253-7f5b-a5d7-09982718fa54" xlink:href="lung-20260930.xsd#lung_CIBCAgreementTrancheBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-70da-8874-9f3d7d54fce6" xlink:to="loc_lung_CIBCAgreementTrancheBMember_01a0cb90-d253-7f5b-a5d7-09982718fa54" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_CIBCAgreementTrancheCMember_01a0cb90-d253-7685-9d08-3097f8042c87" xlink:href="lung-20260930.xsd#lung_CIBCAgreementTrancheCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-70da-8874-9f3d7d54fce6" xlink:to="loc_lung_CIBCAgreementTrancheCMember_01a0cb90-d253-7685-9d08-3097f8042c87" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7161-b0c5-1297e72adfd4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-7a66-b44b-850836e303a1" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7161-b0c5-1297e72adfd4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-7161-b0c5-1297e72adfd4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7161-b0c5-1297e72adfd4" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-7161-b0c5-1297e72adfd4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-75a4-b0b1-564778c1b13b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7161-b0c5-1297e72adfd4" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-75a4-b0b1-564778c1b13b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MediumTermNotesMember_01a0cb90-d253-7b89-885b-8f63a20047be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MediumTermNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-75a4-b0b1-564778c1b13b" xlink:to="loc_us-gaap_MediumTermNotesMember_01a0cb90-d253-7b89-885b-8f63a20047be" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentFaceAmountIncludingAccordionFeature_01a0cb90-d253-7107-a643-e5cc7aa1937c" xlink:href="lung-20260930.xsd#lung_DebtInstrumentFaceAmountIncludingAccordionFeature"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_lung_DebtInstrumentFaceAmountIncludingAccordionFeature_01a0cb90-d253-7107-a643-e5cc7aa1937c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_01a0cb90-d253-78a5-9d30-25d875af7426" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_01a0cb90-d253-78a5-9d30-25d875af7426" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentAccordionFeatureIncreaseFaceAmount_01a0cb90-d253-76fe-9456-57cad018bcf2" xlink:href="lung-20260930.xsd#lung_DebtInstrumentAccordionFeatureIncreaseFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_lung_DebtInstrumentAccordionFeatureIncreaseFaceAmount_01a0cb90-d253-76fe-9456-57cad018bcf2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentPeriodicPaymentInterestOnlyTerm_01a0cb90-d253-78f9-b36f-1c31083e614c" xlink:href="lung-20260930.xsd#lung_DebtInstrumentPeriodicPaymentInterestOnlyTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_lung_DebtInstrumentPeriodicPaymentInterestOnlyTerm_01a0cb90-d253-78f9-b36f-1c31083e614c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentPeriodicPaymentPrincipalAndInterestTerm_01a0cb90-d253-7b41-9446-1d883638eaee" xlink:href="lung-20260930.xsd#lung_DebtInstrumentPeriodicPaymentPrincipalAndInterestTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_lung_DebtInstrumentPeriodicPaymentPrincipalAndInterestTerm_01a0cb90-d253-7b41-9446-1d883638eaee" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_01a0cb90-d253-7d67-bd5e-495bb83b80e7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_01a0cb90-d253-7d67-bd5e-495bb83b80e7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentNewInterestOnlyPeriod_01a0cb90-d253-754c-a132-897baec1885d" xlink:href="lung-20260930.xsd#lung_DebtInstrumentNewInterestOnlyPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_lung_DebtInstrumentNewInterestOnlyPeriod_01a0cb90-d253-754c-a132-897baec1885d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentNewInterestOnlyExtensionPeriod_01a0cb90-d253-76aa-88d8-e1cff36194eb" xlink:href="lung-20260930.xsd#lung_DebtInstrumentNewInterestOnlyExtensionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_lung_DebtInstrumentNewInterestOnlyExtensionPeriod_01a0cb90-d253-76aa-88d8-e1cff36194eb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtCovenantCashAndCashEquivalents_01a0cb90-d253-7816-8b6c-61854bee97f7" xlink:href="lung-20260930.xsd#lung_DebtCovenantCashAndCashEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_lung_DebtCovenantCashAndCashEquivalents_01a0cb90-d253-7816-8b6c-61854bee97f7" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtCovenantTrailingRevenueMinimumPercentage_01a0cb90-d253-7179-998f-eb3b8d4da938" xlink:href="lung-20260930.xsd#lung_DebtCovenantTrailingRevenueMinimumPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_lung_DebtCovenantTrailingRevenueMinimumPercentage_01a0cb90-d253-7179-998f-eb3b8d4da938" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentCovenantCashAndCashEquivalentsMaximumThresholdTriggerForRevenue_01a0cb90-d253-7e88-b324-4973c8678e34" xlink:href="lung-20260930.xsd#lung_DebtInstrumentCovenantCashAndCashEquivalentsMaximumThresholdTriggerForRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_lung_DebtInstrumentCovenantCashAndCashEquivalentsMaximumThresholdTriggerForRevenue_01a0cb90-d253-7e88-b324-4973c8678e34" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentCovenantUnrestrictedCashRequirementPeriod_01a0cb90-d253-7659-865e-639342be6c3a" xlink:href="lung-20260930.xsd#lung_DebtInstrumentCovenantUnrestrictedCashRequirementPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_lung_DebtInstrumentCovenantUnrestrictedCashRequirementPeriod_01a0cb90-d253-7659-865e-639342be6c3a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentMultiplicator_01a0cb90-d253-7399-b7ee-1fd8ec9c3257" xlink:href="lung-20260930.xsd#lung_DebtInstrumentMultiplicator"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_lung_DebtInstrumentMultiplicator_01a0cb90-d253-7399-b7ee-1fd8ec9c3257" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentRefinancingFee_01a0cb90-d253-78fe-8606-e135894b43ab" xlink:href="lung-20260930.xsd#lung_DebtInstrumentRefinancingFee"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_lung_DebtInstrumentRefinancingFee_01a0cb90-d253-78fe-8606-e135894b43ab" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_01a0cb90-d253-7238-942c-bd2e6aec74a7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_01a0cb90-d253-7238-942c-bd2e6aec74a7" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDebtIssuanceCosts_01a0cb90-d253-7723-97db-07de0d600166" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsOfDebtIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_us-gaap_PaymentsOfDebtIssuanceCosts_01a0cb90-d253-7723-97db-07de0d600166" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCostsAndDiscounts_01a0cb90-d253-7e16-87fa-c15d6cecd451" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfFinancingCostsAndDiscounts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_us-gaap_AmortizationOfFinancingCostsAndDiscounts_01a0cb90-d253-7e16-87fa-c15d6cecd451" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebtExcludingAmortization_01a0cb90-d253-7598-aed0-f0ab251b4624" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebtExcludingAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_us-gaap_InterestExpenseDebtExcludingAmortization_01a0cb90-d253-7598-aed0-f0ab251b4624" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfLongTermDebt_01a0cb90-d253-768d-ad62-9cb57f2df27a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_us-gaap_RepaymentsOfLongTermDebt_01a0cb90-d253-768d-ad62-9cb57f2df27a" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentRepaidPrincipal_01a0cb90-d253-7064-b2f5-4c476156b8d9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentRepaidPrincipal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_us-gaap_DebtInstrumentRepaidPrincipal_01a0cb90-d253-7064-b2f5-4c476156b8d9" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_01a0cb90-d253-73a4-a44d-becbdd4ead2f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_us-gaap_InterestExpenseDebt_01a0cb90-d253-73a4-a44d-becbdd4ead2f" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_01a0cb90-d253-708d-bfb1-d29da1741073" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_GainsLossesOnExtinguishmentOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-796b-9dfb-38fb9d850e6c" xlink:to="loc_us-gaap_GainsLossesOnExtinguishmentOfDebt_01a0cb90-d253-708d-bfb1-d29da1741073" xlink:type="arc" order="21"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/LongTermDebtScheduleofLoanComponentsOfDebtDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#LongTermDebtScheduleofLoanComponentsOfDebtDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/LongTermDebtScheduleofLoanComponentsOfDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-7e5e-92b0-6d17094ddc43" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-763b-901a-64805255305b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-7e5e-92b0-6d17094ddc43" xlink:to="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-763b-901a-64805255305b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7a87-a281-07679f378543" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-763b-901a-64805255305b" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7a87-a281-07679f378543" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7a87-a281-07679f378543_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7a87-a281-07679f378543" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7a87-a281-07679f378543_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-72ed-990a-6d1ebdcf91cd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7a87-a281-07679f378543" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-72ed-990a-6d1ebdcf91cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_CIBCAgreementMember_01a0cb90-d253-7c4d-8779-18f1b220c95a" xlink:href="lung-20260930.xsd#lung_CIBCAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-72ed-990a-6d1ebdcf91cd" xlink:to="loc_lung_CIBCAgreementMember_01a0cb90-d253-7c4d-8779-18f1b220c95a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_TheCreditAgreementMember_01a0cb90-d253-78a4-9cbf-94b5e4a481b7" xlink:href="lung-20260930.xsd#lung_TheCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-72ed-990a-6d1ebdcf91cd" xlink:to="loc_lung_TheCreditAgreementMember_01a0cb90-d253-78a4-9cbf-94b5e4a481b7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7a14-89bc-b81378d5bdd3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-763b-901a-64805255305b" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7a14-89bc-b81378d5bdd3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-7a14-89bc-b81378d5bdd3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7a14-89bc-b81378d5bdd3" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-7a14-89bc-b81378d5bdd3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-76c5-908a-ed03edab4c96" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7a14-89bc-b81378d5bdd3" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-76c5-908a-ed03edab4c96" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MediumTermNotesMember_01a0cb90-d253-71fa-8a28-25df8f30b376" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MediumTermNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-76c5-908a-ed03edab4c96" xlink:to="loc_us-gaap_MediumTermNotesMember_01a0cb90-d253-71fa-8a28-25df8f30b376" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_01a0cb90-d253-7894-97af-7cc7671ddd3e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-76c5-908a-ed03edab4c96" xlink:to="loc_us-gaap_LineOfCreditMember_01a0cb90-d253-7894-97af-7cc7671ddd3e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7bbb-b102-a0f414f655f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-763b-901a-64805255305b" xlink:to="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7bbb-b102-a0f414f655f4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-7bbb-b102-a0f414f655f4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7bbb-b102-a0f414f655f4" xlink:to="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-7bbb-b102-a0f414f655f4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-74ed-98e8-678ec611de79" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7bbb-b102-a0f414f655f4" xlink:to="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-74ed-98e8-678ec611de79" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_01a0cb90-d253-711c-ab6d-f60bc0cde884" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-74ed-98e8-678ec611de79" xlink:to="loc_us-gaap_SecuredDebtMember_01a0cb90-d253-711c-ab6d-f60bc0cde884" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_01a0cb90-d253-7e2e-8762-7975afe099db" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-7e5e-92b0-6d17094ddc43" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_01a0cb90-d253-7e2e-8762-7975afe099db" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscount_01a0cb90-d253-7e5e-bd16-5d9d546882c8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentUnamortizedDiscount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-7e5e-92b0-6d17094ddc43" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscount_01a0cb90-d253-7e5e-bd16-5d9d546882c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredFinanceCostsNet_01a0cb90-d253-747a-8f19-5bfdebf773e1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DeferredFinanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-7e5e-92b0-6d17094ddc43" xlink:to="loc_us-gaap_DeferredFinanceCostsNet_01a0cb90-d253-747a-8f19-5bfdebf773e1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebt_01a0cb90-d253-7295-a0b5-bb6f463b9b7b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-7e5e-92b0-6d17094ddc43" xlink:to="loc_us-gaap_LongTermDebt_01a0cb90-d253-7295-a0b5-bb6f463b9b7b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_01a0cb90-d253-7e4f-99b5-99cc450846f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-7e5e-92b0-6d17094ddc43" xlink:to="loc_us-gaap_LongTermDebtCurrent_01a0cb90-d253-7e4f-99b5-99cc450846f6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_01a0cb90-d253-7798-ab87-7d61861b9d03" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-7e5e-92b0-6d17094ddc43" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_01a0cb90-d253-7798-ab87-7d61861b9d03" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/LongTermDebtPerceptiveLoanNarrativeDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#LongTermDebtPerceptiveLoanNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/LongTermDebtPerceptiveLoanNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-7672-bb3f-a6ad223e4d0f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-7672-bb3f-a6ad223e4d0f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7c55-916c-6adde970df92" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-7672-bb3f-a6ad223e4d0f" xlink:to="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7c55-916c-6adde970df92" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-7c55-916c-6adde970df92_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7c55-916c-6adde970df92" xlink:to="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-7c55-916c-6adde970df92_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-7384-8421-93d04a8a2639" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7c55-916c-6adde970df92" xlink:to="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-7384-8421-93d04a8a2639" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_01a0cb90-d253-7ee2-ae9a-022fc3f8cf21" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-7384-8421-93d04a8a2639" xlink:to="loc_us-gaap_SecuredDebtMember_01a0cb90-d253-7ee2-ae9a-022fc3f8cf21" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-72aa-bc78-a3026e319509" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-7672-bb3f-a6ad223e4d0f" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-72aa-bc78-a3026e319509" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-72aa-bc78-a3026e319509_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-72aa-bc78-a3026e319509" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-72aa-bc78-a3026e319509_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7afc-a6a4-14313c18b338" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-72aa-bc78-a3026e319509" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7afc-a6a4-14313c18b338" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_TheCreditAgreementMember_01a0cb90-d253-711e-9019-558b183d941e" xlink:href="lung-20260930.xsd#lung_TheCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7afc-a6a4-14313c18b338" xlink:to="loc_lung_TheCreditAgreementMember_01a0cb90-d253-711e-9019-558b183d941e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_PerceptiveAgreementMember_01a0cb90-d253-7237-8e62-773a24f876fc" xlink:href="lung-20260930.xsd#lung_PerceptiveAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7afc-a6a4-14313c18b338" xlink:to="loc_lung_PerceptiveAgreementMember_01a0cb90-d253-7237-8e62-773a24f876fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-712e-becc-db1d561e791b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-7672-bb3f-a6ad223e4d0f" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-712e-becc-db1d561e791b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-712e-becc-db1d561e791b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-712e-becc-db1d561e791b" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-712e-becc-db1d561e791b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-748c-8c70-291b4d035ae8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-712e-becc-db1d561e791b" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-748c-8c70-291b4d035ae8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_01a0cb90-d253-72a8-b328-d22d7d6ad996" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-748c-8c70-291b4d035ae8" xlink:to="loc_us-gaap_LineOfCreditMember_01a0cb90-d253-72a8-b328-d22d7d6ad996" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentInKindPIKNoteMember_01a0cb90-d253-7285-a319-1fa105865b01" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentInKindPIKNoteMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-748c-8c70-291b4d035ae8" xlink:to="loc_us-gaap_PaymentInKindPIKNoteMember_01a0cb90-d253-7285-a319-1fa105865b01" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_TriggeringEventAxis_01a0cb90-d253-7b05-b779-9d3d18f76a9e" xlink:href="lung-20260930.xsd#lung_TriggeringEventAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-7672-bb3f-a6ad223e4d0f" xlink:to="loc_lung_TriggeringEventAxis_01a0cb90-d253-7b05-b779-9d3d18f76a9e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_TriggeringEventDomain_01a0cb90-d253-7b05-b779-9d3d18f76a9e_default" xlink:href="lung-20260930.xsd#lung_TriggeringEventDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_lung_TriggeringEventAxis_01a0cb90-d253-7b05-b779-9d3d18f76a9e" xlink:to="loc_lung_TriggeringEventDomain_01a0cb90-d253-7b05-b779-9d3d18f76a9e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_TriggeringEventDomain_01a0cb90-d253-7b60-94ec-28c14986ed9b" xlink:href="lung-20260930.xsd#lung_TriggeringEventDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_lung_TriggeringEventAxis_01a0cb90-d253-7b05-b779-9d3d18f76a9e" xlink:to="loc_lung_TriggeringEventDomain_01a0cb90-d253-7b60-94ec-28c14986ed9b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_FirstTrancheMember_01a0cb90-d253-74dd-a6bd-0fe15f1070b3" xlink:href="lung-20260930.xsd#lung_FirstTrancheMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lung_TriggeringEventDomain_01a0cb90-d253-7b60-94ec-28c14986ed9b" xlink:to="loc_lung_FirstTrancheMember_01a0cb90-d253-74dd-a6bd-0fe15f1070b3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_SecondTrancheMember_01a0cb90-d253-79a2-8c22-f57ee02b93e4" xlink:href="lung-20260930.xsd#lung_SecondTrancheMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lung_TriggeringEventDomain_01a0cb90-d253-7b60-94ec-28c14986ed9b" xlink:to="loc_lung_SecondTrancheMember_01a0cb90-d253-79a2-8c22-f57ee02b93e4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_OverTrailing12MonthPeriodEndingMarch312026Member_01a0cb90-d253-76de-826a-5acf4b9b1aed" xlink:href="lung-20260930.xsd#lung_OverTrailing12MonthPeriodEndingMarch312026Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lung_TriggeringEventDomain_01a0cb90-d253-7b60-94ec-28c14986ed9b" xlink:to="loc_lung_OverTrailing12MonthPeriodEndingMarch312026Member_01a0cb90-d253-76de-826a-5acf4b9b1aed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_Trailing12MonthPeriodEndingDecember312030Member_01a0cb90-d253-7385-b7c8-3074f6561271" xlink:href="lung-20260930.xsd#lung_Trailing12MonthPeriodEndingDecember312030Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lung_TriggeringEventDomain_01a0cb90-d253-7b60-94ec-28c14986ed9b" xlink:to="loc_lung_Trailing12MonthPeriodEndingDecember312030Member_01a0cb90-d253-7385-b7c8-3074f6561271" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_01a0cb90-d253-7301-8f7b-057952fe6a6f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-7672-bb3f-a6ad223e4d0f" xlink:to="loc_srt_RangeAxis_01a0cb90-d253-7301-8f7b-057952fe6a6f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a0cb90-d253-7301-8f7b-057952fe6a6f_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_01a0cb90-d253-7301-8f7b-057952fe6a6f" xlink:to="loc_srt_RangeMember_01a0cb90-d253-7301-8f7b-057952fe6a6f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a0cb90-d253-7a74-954e-50a464dbc060" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_01a0cb90-d253-7301-8f7b-057952fe6a6f" xlink:to="loc_srt_RangeMember_01a0cb90-d253-7a74-954e-50a464dbc060" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_01a0cb90-d253-75da-ba94-cfd23db8d9e9" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a0cb90-d253-7a74-954e-50a464dbc060" xlink:to="loc_srt_MinimumMember_01a0cb90-d253-75da-ba94-cfd23db8d9e9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_01a0cb90-d253-71ba-abad-c0df187fc898" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a0cb90-d253-7a74-954e-50a464dbc060" xlink:to="loc_srt_MaximumMember_01a0cb90-d253-71ba-abad-c0df187fc898" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0cb90-d253-700f-9a79-7a09ed95611f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-7672-bb3f-a6ad223e4d0f" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0cb90-d253-700f-9a79-7a09ed95611f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-700f-9a79-7a09ed95611f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0cb90-d253-700f-9a79-7a09ed95611f" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-700f-9a79-7a09ed95611f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-7d51-b2da-05fcfa57b77c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0cb90-d253-700f-9a79-7a09ed95611f" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-7d51-b2da-05fcfa57b77c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_PerceptiveWarrantMember_01a0cb90-d253-7f2d-8296-558ed715851b" xlink:href="lung-20260930.xsd#lung_PerceptiveWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-7d51-b2da-05fcfa57b77c" xlink:to="loc_lung_PerceptiveWarrantMember_01a0cb90-d253-7f2d-8296-558ed715851b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_01a0cb90-d253-7c6f-8414-2e8af4dd2fb5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_01a0cb90-d253-7c6f-8414-2e8af4dd2fb5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_01a0cb90-d253-7dbc-b21e-b15620468c26" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ProceedsFromIssuanceOfLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfLongTermDebt_01a0cb90-d253-7dbc-b21e-b15620468c26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_01a0cb90-d253-77d1-8cff-2932be86786e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_us-gaap_LineOfCreditFacilityRemainingBorrowingCapacity_01a0cb90-d253-77d1-8cff-2932be86786e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_LongTermDebtInstrumentNumberOfDraws_01a0cb90-d253-73c6-bb4a-070b85742f49" xlink:href="lung-20260930.xsd#lung_LongTermDebtInstrumentNumberOfDraws"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_lung_LongTermDebtInstrumentNumberOfDraws_01a0cb90-d253-73c6-bb4a-070b85742f49" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentCovenantRevenueRequirement_01a0cb90-d253-7404-98e8-1ab973c3cbff" xlink:href="lung-20260930.xsd#lung_DebtInstrumentCovenantRevenueRequirement"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_lung_DebtInstrumentCovenantRevenueRequirement_01a0cb90-d253-7404-98e8-1ab973c3cbff" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentApplicableMarginRate_01a0cb90-d253-75ee-b0d6-264377f84742" xlink:href="lung-20260930.xsd#lung_DebtInstrumentApplicableMarginRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_lung_DebtInstrumentApplicableMarginRate_01a0cb90-d253-75ee-b0d6-264377f84742" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_01a0cb90-d253-7606-833a-86eabcfddc51" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_01a0cb90-d253-7606-833a-86eabcfddc51" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentIncreaseDecreaseInMarginRate_01a0cb90-d253-7b44-8243-d9bf3f7d64ee" xlink:href="lung-20260930.xsd#lung_DebtInstrumentIncreaseDecreaseInMarginRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_lung_DebtInstrumentIncreaseDecreaseInMarginRate_01a0cb90-d253-7b44-8243-d9bf3f7d64ee" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_01a0cb90-d253-7124-8cfe-1d1f08b1a3b9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_01a0cb90-d253-7124-8cfe-1d1f08b1a3b9" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentPaidInKindIncreaseToPrincipal_01a0cb90-d253-712d-9ee7-67f1f9ec7f57" xlink:href="lung-20260930.xsd#lung_DebtInstrumentPaidInKindIncreaseToPrincipal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_lung_DebtInstrumentPaidInKindIncreaseToPrincipal_01a0cb90-d253-712d-9ee7-67f1f9ec7f57" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentPrepaymentPremium_01a0cb90-d253-797c-aac1-7e4d5d5474da" xlink:href="lung-20260930.xsd#lung_DebtInstrumentPrepaymentPremium"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_lung_DebtInstrumentPrepaymentPremium_01a0cb90-d253-797c-aac1-7e4d5d5474da" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentExitFeePercentage_01a0cb90-d253-74f5-922f-d78b7b47d89e" xlink:href="lung-20260930.xsd#lung_DebtInstrumentExitFeePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_lung_DebtInstrumentExitFeePercentage_01a0cb90-d253-74f5-922f-d78b7b47d89e" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentCovenantLiquidityAmountMinimum_01a0cb90-d253-758a-8744-029e87b26248" xlink:href="lung-20260930.xsd#lung_DebtInstrumentCovenantLiquidityAmountMinimum"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_lung_DebtInstrumentCovenantLiquidityAmountMinimum_01a0cb90-d253-758a-8744-029e87b26248" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentCovenantConsolidatedRevenueAnnualMinimumAmount_01a0cb90-d253-7300-8397-327173f9d526" xlink:href="lung-20260930.xsd#lung_DebtInstrumentCovenantConsolidatedRevenueAnnualMinimumAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_lung_DebtInstrumentCovenantConsolidatedRevenueAnnualMinimumAmount_01a0cb90-d253-7300-8397-327173f9d526" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsOfDebtIssuanceCosts_01a0cb90-d253-70cd-a6c0-250f77c9dd99" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PaymentsOfDebtIssuanceCosts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_us-gaap_PaymentsOfDebtIssuanceCosts_01a0cb90-d253-70cd-a6c0-250f77c9dd99" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_01a0cb90-d253-781f-a04a-bf696bfe901c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_01a0cb90-d253-781f-a04a-bf696bfe901c" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_01a0cb90-d253-7ea3-a400-e4422a499fe3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_01a0cb90-d253-7ea3-a400-e4422a499fe3" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_01a0cb90-d253-7ef0-a917-ada47148db1e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_01a0cb90-d253-7ef0-a917-ada47148db1e" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebtExcludingAmortization_01a0cb90-d253-7794-b9cb-e29c4f12e710" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_InterestExpenseDebtExcludingAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_us-gaap_InterestExpenseDebtExcludingAmortization_01a0cb90-d253-7794-b9cb-e29c4f12e710" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfFinancingCostsAndDiscounts_01a0cb90-d253-7046-8392-53af2c0586c6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AmortizationOfFinancingCostsAndDiscounts"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-74c5-a3dd-6213c77d2f15" xlink:to="loc_us-gaap_AmortizationOfFinancingCostsAndDiscounts_01a0cb90-d253-7046-8392-53af2c0586c6" xlink:type="arc" order="19"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/LongTermDebtCreditAgreementNarrativeDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#LongTermDebtCreditAgreementNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/LongTermDebtCreditAgreementNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-76ac-860e-05a231877ef9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-752a-8f6d-40dba57246f1" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-76ac-860e-05a231877ef9" xlink:to="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-752a-8f6d-40dba57246f1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7135-9040-e82b8a7e55c9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-752a-8f6d-40dba57246f1" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7135-9040-e82b8a7e55c9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7135-9040-e82b8a7e55c9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7135-9040-e82b8a7e55c9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7135-9040-e82b8a7e55c9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7c55-ba57-aa350d264e25" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7135-9040-e82b8a7e55c9" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7c55-ba57-aa350d264e25" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_COVID19CreditAgreementMember_01a0cb90-d253-735b-9a20-35854342eccd" xlink:href="lung-20260930.xsd#lung_COVID19CreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7c55-ba57-aa350d264e25" xlink:to="loc_lung_COVID19CreditAgreementMember_01a0cb90-d253-735b-9a20-35854342eccd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-753e-8bba-43a8be6dab5c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-752a-8f6d-40dba57246f1" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-753e-8bba-43a8be6dab5c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-753e-8bba-43a8be6dab5c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-753e-8bba-43a8be6dab5c" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-753e-8bba-43a8be6dab5c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-75c3-8f1b-a447b54ffce0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-753e-8bba-43a8be6dab5c" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-75c3-8f1b-a447b54ffce0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_01a0cb90-d253-7989-a7dd-6809de2738de" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-75c3-8f1b-a447b54ffce0" xlink:to="loc_us-gaap_LineOfCreditMember_01a0cb90-d253-7989-a7dd-6809de2738de" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_01a0cb90-d253-70e5-ab60-01e08b4ed2c9" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_01a0cb90-d253-752a-8f6d-40dba57246f1" xlink:to="loc_dei_LegalEntityAxis_01a0cb90-d253-70e5-ab60-01e08b4ed2c9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_01a0cb90-d253-70e5-ab60-01e08b4ed2c9_default" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_01a0cb90-d253-70e5-ab60-01e08b4ed2c9" xlink:to="loc_dei_EntityDomain_01a0cb90-d253-70e5-ab60-01e08b4ed2c9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_01a0cb90-d253-7bc1-849b-cd530e3a5077" xlink:href="https://xbrl.sec.gov/dei/2026/dei-2026.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_01a0cb90-d253-70e5-ab60-01e08b4ed2c9" xlink:to="loc_dei_EntityDomain_01a0cb90-d253-7bc1-849b-cd530e3a5077" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_PulmonxInternationalSarlMember_01a0cb90-d253-78d8-bffc-c4d5f0cae532" xlink:href="lung-20260930.xsd#lung_PulmonxInternationalSarlMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_01a0cb90-d253-7bc1-849b-cd530e3a5077" xlink:to="loc_lung_PulmonxInternationalSarlMember_01a0cb90-d253-78d8-bffc-c4d5f0cae532" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_01a0cb90-d253-78e2-9d11-ab1084120bf3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-76ac-860e-05a231877ef9" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_01a0cb90-d253-78e2-9d11-ab1084120bf3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_01a0cb90-d253-78fe-b4b7-8563dbfb4b4f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-76ac-860e-05a231877ef9" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_01a0cb90-d253-78fe-b4b7-8563dbfb4b4f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_DebtInstrumentNumberOfInstallments_01a0cb90-d253-708d-a493-fc5d5bfc5f5c" xlink:href="lung-20260930.xsd#lung_DebtInstrumentNumberOfInstallments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-76ac-860e-05a231877ef9" xlink:to="loc_lung_DebtInstrumentNumberOfInstallments_01a0cb90-d253-708d-a493-fc5d5bfc5f5c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfDebt_01a0cb90-d253-705e-bac9-82ec453ff232" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RepaymentsOfDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_01a0cb90-d253-76ac-860e-05a231877ef9" xlink:to="loc_us-gaap_RepaymentsOfDebt_01a0cb90-d253-705e-bac9-82ec453ff232" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/CommonStockWarrantLiabilityDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#CommonStockWarrantLiabilityDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/CommonStockWarrantLiabilityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightLineItems_01a0cb90-d253-715a-81d0-9eb0ed5bb623" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightTable_01a0cb90-d253-71f6-bec8-052a36d2e8d4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfWarrantOrRightLineItems_01a0cb90-d253-715a-81d0-9eb0ed5bb623" xlink:to="loc_us-gaap_ClassOfWarrantOrRightTable_01a0cb90-d253-71f6-bec8-052a36d2e8d4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_01a0cb90-d253-7320-8446-64fcc7d170fb" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ClassOfWarrantOrRightTable_01a0cb90-d253-71f6-bec8-052a36d2e8d4" xlink:to="loc_srt_RangeAxis_01a0cb90-d253-7320-8446-64fcc7d170fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a0cb90-d253-7320-8446-64fcc7d170fb_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_01a0cb90-d253-7320-8446-64fcc7d170fb" xlink:to="loc_srt_RangeMember_01a0cb90-d253-7320-8446-64fcc7d170fb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a0cb90-d253-7f5a-851d-5893985d5f5f" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_01a0cb90-d253-7320-8446-64fcc7d170fb" xlink:to="loc_srt_RangeMember_01a0cb90-d253-7f5a-851d-5893985d5f5f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_01a0cb90-d253-7c2e-8c76-a063a13bef43" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a0cb90-d253-7f5a-851d-5893985d5f5f" xlink:to="loc_srt_MaximumMember_01a0cb90-d253-7c2e-8c76-a063a13bef43" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAdjustmentOfWarrants_01a0cb90-d253-767b-b277-b5a55ad5eff3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_FairValueAdjustmentOfWarrants"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightLineItems_01a0cb90-d253-715a-81d0-9eb0ed5bb623" xlink:to="loc_us-gaap_FairValueAdjustmentOfWarrants_01a0cb90-d253-767b-b277-b5a55ad5eff3" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#CommitmentsandContingenciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_01a0cb90-d253-72fb-81c5-7a7187e70f27" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_01a0cb90-d253-77ee-b40f-74fecf66dc67" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_01a0cb90-d253-72fb-81c5-7a7187e70f27" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_01a0cb90-d253-77ee-b40f-74fecf66dc67" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis_01a0cb90-d253-75d8-90ac-c89f721dcb4c" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_01a0cb90-d253-77ee-b40f-74fecf66dc67" xlink:to="loc_srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis_01a0cb90-d253-75d8-90ac-c89f721dcb4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MortgageLoansOnRealEstateNamePropertyTypeDomain_01a0cb90-d253-75d8-90ac-c89f721dcb4c_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MortgageLoansOnRealEstateNamePropertyTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis_01a0cb90-d253-75d8-90ac-c89f721dcb4c" xlink:to="loc_srt_MortgageLoansOnRealEstateNamePropertyTypeDomain_01a0cb90-d253-75d8-90ac-c89f721dcb4c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MortgageLoansOnRealEstateNamePropertyTypeDomain_01a0cb90-d253-7e04-9ed8-b0b4cf17e7e0" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MortgageLoansOnRealEstateNamePropertyTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MortgageLoansOnRealEstateDescriptionTypeOfPropertyAxis_01a0cb90-d253-75d8-90ac-c89f721dcb4c" xlink:to="loc_srt_MortgageLoansOnRealEstateNamePropertyTypeDomain_01a0cb90-d253-7e04-9ed8-b0b4cf17e7e0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_OfficeBuildingMember_01a0cb90-d253-7cb8-8895-4a2cb0eca21d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_OfficeBuildingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_MortgageLoansOnRealEstateNamePropertyTypeDomain_01a0cb90-d253-7e04-9ed8-b0b4cf17e7e0" xlink:to="loc_srt_OfficeBuildingMember_01a0cb90-d253-7cb8-8895-4a2cb0eca21d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_OfficeBuildingExistingPremisesMember_01a0cb90-d253-7b73-9fce-44fc0666a728" xlink:href="lung-20260930.xsd#lung_OfficeBuildingExistingPremisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OfficeBuildingMember_01a0cb90-d253-7cb8-8895-4a2cb0eca21d" xlink:to="loc_lung_OfficeBuildingExistingPremisesMember_01a0cb90-d253-7b73-9fce-44fc0666a728" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_OfficeBuildingExpansionPremisesMember_01a0cb90-d253-778d-bf7c-f61a4e92cdc4" xlink:href="lung-20260930.xsd#lung_OfficeBuildingExpansionPremisesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_OfficeBuildingMember_01a0cb90-d253-7cb8-8895-4a2cb0eca21d" xlink:to="loc_lung_OfficeBuildingExpansionPremisesMember_01a0cb90-d253-778d-bf7c-f61a4e92cdc4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_01a0cb90-d253-7a73-977a-e1d8a3fdf4fe" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_01a0cb90-d253-77ee-b40f-74fecf66dc67" xlink:to="loc_srt_StatementGeographicalAxis_01a0cb90-d253-7a73-977a-e1d8a3fdf4fe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7a73-977a-e1d8a3fdf4fe_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_01a0cb90-d253-7a73-977a-e1d8a3fdf4fe" xlink:to="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7a73-977a-e1d8a3fdf4fe_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7ec3-a9f8-b71a43181ee1" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_01a0cb90-d253-7a73-977a-e1d8a3fdf4fe" xlink:to="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7ec3-a9f8-b71a43181ee1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_stpr_CA_01a0cb90-d253-7c63-a5ba-b4c2ca64dd7f" xlink:href="https://xbrl.sec.gov/stpr/2026/stpr-2026.xsd#stpr_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7ec3-a9f8-b71a43181ee1" xlink:to="loc_stpr_CA_01a0cb90-d253-7c63-a5ba-b4c2ca64dd7f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CH_01a0cb90-d253-7d76-996e-106a0d690e24" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_CH"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7ec3-a9f8-b71a43181ee1" xlink:to="loc_country_CH_01a0cb90-d253-7d76-996e-106a0d690e24" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_TriggeringEventTypeAxis_01a0cb90-d253-7728-8f09-154f872f08e8" xlink:href="lung-20260930.xsd#lung_TriggeringEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_01a0cb90-d253-77ee-b40f-74fecf66dc67" xlink:to="loc_lung_TriggeringEventTypeAxis_01a0cb90-d253-7728-8f09-154f872f08e8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_TriggeringEventTypeDomain_01a0cb90-d253-7728-8f09-154f872f08e8_default" xlink:href="lung-20260930.xsd#lung_TriggeringEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_lung_TriggeringEventTypeAxis_01a0cb90-d253-7728-8f09-154f872f08e8" xlink:to="loc_lung_TriggeringEventTypeDomain_01a0cb90-d253-7728-8f09-154f872f08e8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_TriggeringEventTypeDomain_01a0cb90-d253-722e-89bb-c40ad092ea9f" xlink:href="lung-20260930.xsd#lung_TriggeringEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_lung_TriggeringEventTypeAxis_01a0cb90-d253-7728-8f09-154f872f08e8" xlink:to="loc_lung_TriggeringEventTypeDomain_01a0cb90-d253-722e-89bb-c40ad092ea9f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_MonthlyFromMarch12025Member_01a0cb90-d253-798c-96f0-bc9dd189ed51" xlink:href="lung-20260930.xsd#lung_MonthlyFromMarch12025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lung_TriggeringEventTypeDomain_01a0cb90-d253-722e-89bb-c40ad092ea9f" xlink:to="loc_lung_MonthlyFromMarch12025Member_01a0cb90-d253-798c-96f0-bc9dd189ed51" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_MonthlyFromDecember12025Member_01a0cb90-d253-73cd-aede-f90826d9d8c3" xlink:href="lung-20260930.xsd#lung_MonthlyFromDecember12025Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lung_TriggeringEventTypeDomain_01a0cb90-d253-722e-89bb-c40ad092ea9f" xlink:to="loc_lung_MonthlyFromDecember12025Member_01a0cb90-d253-73cd-aede-f90826d9d8c3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_MonthlyFromJune12028Member_01a0cb90-d253-70e7-adc4-bafac2b7d967" xlink:href="lung-20260930.xsd#lung_MonthlyFromJune12028Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_lung_TriggeringEventTypeDomain_01a0cb90-d253-722e-89bb-c40ad092ea9f" xlink:to="loc_lung_MonthlyFromJune12028Member_01a0cb90-d253-70e7-adc4-bafac2b7d967" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-73b3-a5fc-b4d219162de8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_01a0cb90-d253-77ee-b40f-74fecf66dc67" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-73b3-a5fc-b4d219162de8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-73b3-a5fc-b4d219162de8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-73b3-a5fc-b4d219162de8" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-73b3-a5fc-b4d219162de8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-70aa-b206-52190c75cf68" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_01a0cb90-d253-73b3-a5fc-b4d219162de8" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-70aa-b206-52190c75cf68" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingMember_01a0cb90-d253-74e5-aa8a-ea898ec048b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_BuildingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-70aa-b206-52190c75cf68" xlink:to="loc_us-gaap_BuildingMember_01a0cb90-d253-74e5-aa8a-ea898ec048b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VehiclesMember_01a0cb90-d253-7c88-928e-98d56528b9bf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VehiclesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_01a0cb90-d253-70aa-b206-52190c75cf68" xlink:to="loc_us-gaap_VehiclesMember_01a0cb90-d253-7c88-928e-98d56528b9bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_NetLeasedArea_01a0cb90-d253-7505-9cb3-ddf608a57c99" xlink:href="lung-20260930.xsd#lung_NetLeasedArea"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_01a0cb90-d253-72fb-81c5-7a7187e70f27" xlink:to="loc_lung_NetLeasedArea_01a0cb90-d253-7505-9cb3-ddf608a57c99" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_NetSubleasedArea_01a0cb90-d253-7c94-9260-16faeae04d09" xlink:href="lung-20260930.xsd#lung_NetSubleasedArea"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_01a0cb90-d253-72fb-81c5-7a7187e70f27" xlink:to="loc_lung_NetSubleasedArea_01a0cb90-d253-7c94-9260-16faeae04d09" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeasePayments_01a0cb90-d253-765b-9833-22a7b0fb48f4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_OperatingLeasePayments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_01a0cb90-d253-72fb-81c5-7a7187e70f27" xlink:to="loc_us-gaap_OperatingLeasePayments_01a0cb90-d253-765b-9833-22a7b0fb48f4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_LesseeOperatingLeaseExpenseAnnualIncreaseRate_01a0cb90-d253-7c71-bbc6-3df071c42cfc" xlink:href="lung-20260930.xsd#lung_LesseeOperatingLeaseExpenseAnnualIncreaseRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_01a0cb90-d253-72fb-81c5-7a7187e70f27" xlink:to="loc_lung_LesseeOperatingLeaseExpenseAnnualIncreaseRate_01a0cb90-d253-7c71-bbc6-3df071c42cfc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_LesseeOperatingLeaseExtensionOptions_01a0cb90-d253-7827-ae5b-57acb854bdd8" xlink:href="lung-20260930.xsd#lung_LesseeOperatingLeaseExtensionOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_01a0cb90-d253-72fb-81c5-7a7187e70f27" xlink:to="loc_lung_LesseeOperatingLeaseExtensionOptions_01a0cb90-d253-7827-ae5b-57acb854bdd8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseTermOfContract_01a0cb90-d253-74dc-b3f3-4b3041d1d06b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeOperatingLeaseTermOfContract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_01a0cb90-d253-72fb-81c5-7a7187e70f27" xlink:to="loc_us-gaap_LesseeOperatingLeaseTermOfContract_01a0cb90-d253-74dc-b3f3-4b3041d1d06b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability_01a0cb90-d253-7694-96bf-c2bdd11e23f6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_01a0cb90-d253-72fb-81c5-7a7187e70f27" xlink:to="loc_us-gaap_RightOfUseAssetObtainedInExchangeForFinanceLeaseLiability_01a0cb90-d253-7694-96bf-c2bdd11e23f6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_01a0cb90-d253-7be5-9c28-8e2321159c9b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_01a0cb90-d253-72fb-81c5-7a7187e70f27" xlink:to="loc_us-gaap_LesseeOperatingLeaseLiabilityPaymentsDue_01a0cb90-d253-7be5-9c28-8e2321159c9b" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_NumberOfVehicleLeases_01a0cb90-d253-7ffb-9ff3-c004a2ebc9a0" xlink:href="lung-20260930.xsd#lung_NumberOfVehicleLeases"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_01a0cb90-d253-72fb-81c5-7a7187e70f27" xlink:to="loc_lung_NumberOfVehicleLeases_01a0cb90-d253-7ffb-9ff3-c004a2ebc9a0" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/StockholdersEquityNarrativeDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#StockholdersEquityNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/StockholdersEquityNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7b9c-b3ab-821adc853f97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7b9c-b3ab-821adc853f97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7b9c-b3ab-821adc853f97_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7b9c-b3ab-821adc853f97" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7b9c-b3ab-821adc853f97_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7b9c-b3ab-821adc853f97" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_01a0cb90-d253-7d07-a60d-88beebdddc56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_01a0cb90-d253-7d07-a60d-88beebdddc56" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_01a0cb90-d253-76d0-a379-1de03728df21" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:to="loc_us-gaap_PerformanceSharesMember_01a0cb90-d253-76d0-a379-1de03728df21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_MarketBasedRestrictedStockUnitsMember_01a0cb90-d253-7475-b128-7911b5dacfb4" xlink:href="lung-20260930.xsd#lung_MarketBasedRestrictedStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:to="loc_lung_MarketBasedRestrictedStockUnitsMember_01a0cb90-d253-7475-b128-7911b5dacfb4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_StockOptionsAndRestrictedStockUnitsMember_01a0cb90-d253-7f73-b34f-18828ef95b18" xlink:href="lung-20260930.xsd#lung_StockOptionsAndRestrictedStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:to="loc_lung_StockOptionsAndRestrictedStockUnitsMember_01a0cb90-d253-7f73-b34f-18828ef95b18" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_01a0cb90-d253-757c-bd7d-fa8b7130b945" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:to="loc_us-gaap_EmployeeStockMember_01a0cb90-d253-757c-bd7d-fa8b7130b945" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_01a0cb90-d253-7c2a-b8ba-034da2124dce" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_srt_RangeAxis_01a0cb90-d253-7c2a-b8ba-034da2124dce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a0cb90-d253-7c2a-b8ba-034da2124dce_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_01a0cb90-d253-7c2a-b8ba-034da2124dce" xlink:to="loc_srt_RangeMember_01a0cb90-d253-7c2a-b8ba-034da2124dce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a0cb90-d253-7639-ba02-4529f443e7ff" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_01a0cb90-d253-7c2a-b8ba-034da2124dce" xlink:to="loc_srt_RangeMember_01a0cb90-d253-7639-ba02-4529f443e7ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_01a0cb90-d253-7da5-8b71-6eef91318462" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a0cb90-d253-7639-ba02-4529f443e7ff" xlink:to="loc_srt_MinimumMember_01a0cb90-d253-7da5-8b71-6eef91318462" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_01a0cb90-d253-7944-b371-e5a763ecb7ed" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a0cb90-d253-7639-ba02-4529f443e7ff" xlink:to="loc_srt_MaximumMember_01a0cb90-d253-7944-b371-e5a763ecb7ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_01a0cb90-d253-7bb5-a482-3a21bdc05050" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_VestingAxis_01a0cb90-d253-7bb5-a482-3a21bdc05050" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_01a0cb90-d253-7bb5-a482-3a21bdc05050_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_01a0cb90-d253-7bb5-a482-3a21bdc05050" xlink:to="loc_us-gaap_VestingDomain_01a0cb90-d253-7bb5-a482-3a21bdc05050_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_01a0cb90-d253-7f88-a446-a665e5d70a1b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_01a0cb90-d253-7bb5-a482-3a21bdc05050" xlink:to="loc_us-gaap_VestingDomain_01a0cb90-d253-7f88-a446-a665e5d70a1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_01a0cb90-d253-7ab4-83a5-dc12d51e1826" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_01a0cb90-d253-7f88-a446-a665e5d70a1b" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_01a0cb90-d253-7ab4-83a5-dc12d51e1826" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_01a0cb90-d253-71e7-bbe4-f99d3c822ade" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_01a0cb90-d253-71e7-bbe4-f99d3c822ade" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0cb90-d253-704a-8cfd-8439d4c18125" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0cb90-d253-704a-8cfd-8439d4c18125" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-704a-8cfd-8439d4c18125_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0cb90-d253-704a-8cfd-8439d4c18125" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-704a-8cfd-8439d4c18125_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-7e5d-88c7-8996e5195ca6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0cb90-d253-704a-8cfd-8439d4c18125" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-7e5d-88c7-8996e5195ca6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_PerceptiveWarrantMember_01a0cb90-d253-7edd-befc-43a4abce07f3" xlink:href="lung-20260930.xsd#lung_PerceptiveWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-7e5d-88c7-8996e5195ca6" xlink:to="loc_lung_PerceptiveWarrantMember_01a0cb90-d253-7edd-befc-43a4abce07f3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_InitialWarrantsMember_01a0cb90-d253-7edd-b74a-e06ada4e09d0" xlink:href="lung-20260930.xsd#lung_InitialWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-7e5d-88c7-8996e5195ca6" xlink:to="loc_lung_InitialWarrantsMember_01a0cb90-d253-7edd-b74a-e06ada4e09d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7faa-b7be-32dcdc638a4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7faa-b7be-32dcdc638a4e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-7faa-b7be-32dcdc638a4e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7faa-b7be-32dcdc638a4e" xlink:to="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-7faa-b7be-32dcdc638a4e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-728d-9b54-a48ff5fe292f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7faa-b7be-32dcdc638a4e" xlink:to="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-728d-9b54-a48ff5fe292f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_01a0cb90-d253-7a72-b33e-1407c9c20cc5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-728d-9b54-a48ff5fe292f" xlink:to="loc_us-gaap_SecuredDebtMember_01a0cb90-d253-7a72-b33e-1407c9c20cc5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7577-892d-514997d9a5e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7577-892d-514997d9a5e0" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7577-892d-514997d9a5e0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7577-892d-514997d9a5e0" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7577-892d-514997d9a5e0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7ded-aeb0-feb800d159bb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7577-892d-514997d9a5e0" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7ded-aeb0-feb800d159bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_TheCreditAgreementMember_01a0cb90-d253-7b4d-aa1b-17f6e5c6c04d" xlink:href="lung-20260930.xsd#lung_TheCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7ded-aeb0-feb800d159bb" xlink:to="loc_lung_TheCreditAgreementMember_01a0cb90-d253-7b4d-aa1b-17f6e5c6c04d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7555-9cce-a6bca4a6cd32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7555-9cce-a6bca4a6cd32" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-7555-9cce-a6bca4a6cd32_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7555-9cce-a6bca4a6cd32" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-7555-9cce-a6bca4a6cd32_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-76ac-b0e8-b511fdbe530b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7555-9cce-a6bca4a6cd32" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-76ac-b0e8-b511fdbe530b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_01a0cb90-d253-77b9-84a9-c5c9db791ce5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-76ac-b0e8-b511fdbe530b" xlink:to="loc_us-gaap_LineOfCreditMember_01a0cb90-d253-77b9-84a9-c5c9db791ce5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_01a0cb90-d253-7d36-ad6c-57af2534165c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_01a0cb90-d253-7d36-ad6c-57af2534165c" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7d36-ad6c-57af2534165c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_01a0cb90-d253-7d36-ad6c-57af2534165c" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7d36-ad6c-57af2534165c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7b3b-af91-9ac04336de6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_01a0cb90-d253-7d36-ad6c-57af2534165c" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7b3b-af91-9ac04336de6d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedTermMember_01a0cb90-d253-7235-9126-394af1708580" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputExpectedTermMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7b3b-af91-9ac04336de6d" xlink:to="loc_us-gaap_MeasurementInputExpectedTermMember_01a0cb90-d253-7235-9126-394af1708580" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputPriceVolatilityMember_01a0cb90-d253-7e0a-abf9-3c25f1e38da6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputPriceVolatilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7b3b-af91-9ac04336de6d" xlink:to="loc_us-gaap_MeasurementInputPriceVolatilityMember_01a0cb90-d253-7e0a-abf9-3c25f1e38da6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_01a0cb90-d253-727a-9e02-50a2cfd55d4f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7b3b-af91-9ac04336de6d" xlink:to="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_01a0cb90-d253-727a-9e02-50a2cfd55d4f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedDividendRateMember_01a0cb90-d253-74d4-bb35-db863d14ede9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputExpectedDividendRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7b3b-af91-9ac04336de6d" xlink:to="loc_us-gaap_MeasurementInputExpectedDividendRateMember_01a0cb90-d253-74d4-bb35-db863d14ede9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_01a0cb90-d253-7a05-ad91-edd08b4ff068" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_01a0cb90-d253-7a05-ad91-edd08b4ff068" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCommonStock_01a0cb90-d253-7a81-9bed-440903a94bf7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_DividendsCommonStock_01a0cb90-d253-7a81-9bed-440903a94bf7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_NumberOfVotesForEachShareOfCommonStockHeld_01a0cb90-d253-73e3-9673-ce4112c6bb8d" xlink:href="lung-20260930.xsd#lung_NumberOfVotesForEachShareOfCommonStockHeld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_lung_NumberOfVotesForEachShareOfCommonStockHeld_01a0cb90-d253-73e3-9673-ce4112c6bb8d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_01a0cb90-d253-79b8-af51-4847a89dbcb8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_01a0cb90-d253-79b8-af51-4847a89dbcb8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_01a0cb90-d253-7aa1-b75c-bcd742ede923" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_01a0cb90-d253-7aa1-b75c-bcd742ede923" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue_01a0cb90-d253-7ce3-b211-363c7c953cb9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue_01a0cb90-d253-7ce3-b211-363c7c953cb9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_01a0cb90-d253-7624-861a-4b76ae9fafce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_01a0cb90-d253-7624-861a-4b76ae9fafce" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_01a0cb90-d253-7b71-a548-fafb043c3b61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_01a0cb90-d253-7b71-a548-fafb043c3b61" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_01a0cb90-d253-722f-8e62-473c5a35c9b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_01a0cb90-d253-722f-8e62-473c5a35c9b6" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_01a0cb90-d253-75c6-9198-4f5fa45a2f5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_01a0cb90-d253-75c6-9198-4f5fa45a2f5b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_01a0cb90-d253-7fcf-808a-63c18b624a8c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_01a0cb90-d253-7fcf-808a-63c18b624a8c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharePrice_01a0cb90-d253-7519-95ba-5291b4531095" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_SharePrice_01a0cb90-d253-7519-95ba-5291b4531095" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_ShareBasedCompensationArrangementByShareBasedPaymentAwardTradingPeriod_01a0cb90-d253-7b6a-94d8-574ef8b27875" xlink:href="lung-20260930.xsd#lung_ShareBasedCompensationArrangementByShareBasedPaymentAwardTradingPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_lung_ShareBasedCompensationArrangementByShareBasedPaymentAwardTradingPeriod_01a0cb90-d253-7b6a-94d8-574ef8b27875" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_01a0cb90-d253-73ae-a175-d437c0dd9bc6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_01a0cb90-d253-73ae-a175-d437c0dd9bc6" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_01a0cb90-d253-7e43-bd39-5ecdac910c9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_01a0cb90-d253-7e43-bd39-5ecdac910c9d" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_01a0cb90-d253-7434-8002-0f01f302ba8b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_01a0cb90-d253-7434-8002-0f01f302ba8b" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCostOfEquity_01a0cb90-d253-76ee-84a3-465e0377394b" xlink:href="lung-20260930.xsd#lung_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCostOfEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_lung_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCostOfEquity_01a0cb90-d253-76ee-84a3-465e0377394b" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_01a0cb90-d253-7cc5-9d20-bf1f98c8aedd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_01a0cb90-d253-7cc5-9d20-bf1f98c8aedd" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_01a0cb90-d253-7243-98d5-b3edc389ae31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_01a0cb90-d253-7243-98d5-b3edc389ae31" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_01a0cb90-d253-734b-bd68-e18d332174c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_01a0cb90-d253-734b-bd68-e18d332174c2" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_01a0cb90-d253-7140-b325-0939cae3e69e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_01a0cb90-d253-7140-b325-0939cae3e69e" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_01a0cb90-d253-7513-a4c6-b112574eb746" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_01a0cb90-d253-7513-a4c6-b112574eb746" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_01a0cb90-d253-7566-9331-44736798a377" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_01a0cb90-d253-7566-9331-44736798a377" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_01a0cb90-d253-7d09-831d-b15d587adc02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_01a0cb90-d253-7d09-831d-b15d587adc02" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_01a0cb90-d253-74dd-abb2-98452fefcc6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_01a0cb90-d253-74dd-abb2-98452fefcc6f" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_01a0cb90-d253-7b1c-95e8-90d9bc5d8f64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstandingTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_01a0cb90-d253-7b1c-95e8-90d9bc5d8f64" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_01a0cb90-d253-71a2-825e-4708e273490e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstandingMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_01a0cb90-d253-71a2-825e-4708e273490e" xlink:type="arc" order="26"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/StockholdersEquityNarrativeDetails_1" xlink:type="simple" xlink:href="lung-20260930.xsd#StockholdersEquityNarrativeDetails_1"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/StockholdersEquityNarrativeDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7b9c-b3ab-821adc853f97" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7b9c-b3ab-821adc853f97" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7b9c-b3ab-821adc853f97_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7b9c-b3ab-821adc853f97" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7b9c-b3ab-821adc853f97_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7b9c-b3ab-821adc853f97" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_01a0cb90-d253-7d07-a60d-88beebdddc56" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_01a0cb90-d253-7d07-a60d-88beebdddc56" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_01a0cb90-d253-76d0-a379-1de03728df21" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:to="loc_us-gaap_PerformanceSharesMember_01a0cb90-d253-76d0-a379-1de03728df21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_MarketBasedRestrictedStockUnitsMember_01a0cb90-d253-7475-b128-7911b5dacfb4" xlink:href="lung-20260930.xsd#lung_MarketBasedRestrictedStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:to="loc_lung_MarketBasedRestrictedStockUnitsMember_01a0cb90-d253-7475-b128-7911b5dacfb4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_StockOptionsAndRestrictedStockUnitsMember_01a0cb90-d253-7f73-b34f-18828ef95b18" xlink:href="lung-20260930.xsd#lung_StockOptionsAndRestrictedStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:to="loc_lung_StockOptionsAndRestrictedStockUnitsMember_01a0cb90-d253-7f73-b34f-18828ef95b18" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_01a0cb90-d253-757c-bd7d-fa8b7130b945" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-76a1-b720-528c6d4da86b" xlink:to="loc_us-gaap_EmployeeStockMember_01a0cb90-d253-757c-bd7d-fa8b7130b945" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_01a0cb90-d253-7c2a-b8ba-034da2124dce" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_srt_RangeAxis_01a0cb90-d253-7c2a-b8ba-034da2124dce" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a0cb90-d253-7c2a-b8ba-034da2124dce_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_01a0cb90-d253-7c2a-b8ba-034da2124dce" xlink:to="loc_srt_RangeMember_01a0cb90-d253-7c2a-b8ba-034da2124dce_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_01a0cb90-d253-7639-ba02-4529f443e7ff" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_01a0cb90-d253-7c2a-b8ba-034da2124dce" xlink:to="loc_srt_RangeMember_01a0cb90-d253-7639-ba02-4529f443e7ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_01a0cb90-d253-7da5-8b71-6eef91318462" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a0cb90-d253-7639-ba02-4529f443e7ff" xlink:to="loc_srt_MinimumMember_01a0cb90-d253-7da5-8b71-6eef91318462" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_01a0cb90-d253-7944-b371-e5a763ecb7ed" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_01a0cb90-d253-7639-ba02-4529f443e7ff" xlink:to="loc_srt_MaximumMember_01a0cb90-d253-7944-b371-e5a763ecb7ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_01a0cb90-d253-7bb5-a482-3a21bdc05050" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_VestingAxis_01a0cb90-d253-7bb5-a482-3a21bdc05050" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_01a0cb90-d253-7bb5-a482-3a21bdc05050_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_01a0cb90-d253-7bb5-a482-3a21bdc05050" xlink:to="loc_us-gaap_VestingDomain_01a0cb90-d253-7bb5-a482-3a21bdc05050_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_01a0cb90-d253-7f88-a446-a665e5d70a1b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_01a0cb90-d253-7bb5-a482-3a21bdc05050" xlink:to="loc_us-gaap_VestingDomain_01a0cb90-d253-7f88-a446-a665e5d70a1b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_01a0cb90-d253-7ab4-83a5-dc12d51e1826" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_01a0cb90-d253-7f88-a446-a665e5d70a1b" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_01a0cb90-d253-7ab4-83a5-dc12d51e1826" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0cb90-d253-704a-8cfd-8439d4c18125" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0cb90-d253-704a-8cfd-8439d4c18125" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-704a-8cfd-8439d4c18125_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0cb90-d253-704a-8cfd-8439d4c18125" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-704a-8cfd-8439d4c18125_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-7e5d-88c7-8996e5195ca6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ClassOfWarrantOrRightAxis_01a0cb90-d253-704a-8cfd-8439d4c18125" xlink:to="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-7e5d-88c7-8996e5195ca6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_PerceptiveWarrantMember_01a0cb90-d253-7edd-befc-43a4abce07f3" xlink:href="lung-20260930.xsd#lung_PerceptiveWarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-7e5d-88c7-8996e5195ca6" xlink:to="loc_lung_PerceptiveWarrantMember_01a0cb90-d253-7edd-befc-43a4abce07f3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_InitialWarrantsMember_01a0cb90-d253-7edd-b74a-e06ada4e09d0" xlink:href="lung-20260930.xsd#lung_InitialWarrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfWarrantOrRightDomain_01a0cb90-d253-7e5d-88c7-8996e5195ca6" xlink:to="loc_lung_InitialWarrantsMember_01a0cb90-d253-7edd-b74a-e06ada4e09d0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7faa-b7be-32dcdc638a4e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7faa-b7be-32dcdc638a4e" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-7faa-b7be-32dcdc638a4e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7faa-b7be-32dcdc638a4e" xlink:to="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-7faa-b7be-32dcdc638a4e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-728d-9b54-a48ff5fe292f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_01a0cb90-d253-7faa-b7be-32dcdc638a4e" xlink:to="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-728d-9b54-a48ff5fe292f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SecuredDebtMember_01a0cb90-d253-7a72-b33e-1407c9c20cc5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SecuredDebtMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_01a0cb90-d253-728d-9b54-a48ff5fe292f" xlink:to="loc_us-gaap_SecuredDebtMember_01a0cb90-d253-7a72-b33e-1407c9c20cc5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7577-892d-514997d9a5e0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7577-892d-514997d9a5e0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7577-892d-514997d9a5e0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7577-892d-514997d9a5e0" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7577-892d-514997d9a5e0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7ded-aeb0-feb800d159bb" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_01a0cb90-d253-7577-892d-514997d9a5e0" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7ded-aeb0-feb800d159bb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_TheCreditAgreementMember_01a0cb90-d253-7b4d-aa1b-17f6e5c6c04d" xlink:href="lung-20260930.xsd#lung_TheCreditAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_01a0cb90-d253-7ded-aeb0-feb800d159bb" xlink:to="loc_lung_TheCreditAgreementMember_01a0cb90-d253-7b4d-aa1b-17f6e5c6c04d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7555-9cce-a6bca4a6cd32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7555-9cce-a6bca4a6cd32" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-7555-9cce-a6bca4a6cd32_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7555-9cce-a6bca4a6cd32" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-7555-9cce-a6bca4a6cd32_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-76ac-b0e8-b511fdbe530b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_01a0cb90-d253-7555-9cce-a6bca4a6cd32" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-76ac-b0e8-b511fdbe530b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditMember_01a0cb90-d253-77b9-84a9-c5c9db791ce5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_LineOfCreditMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_01a0cb90-d253-76ac-b0e8-b511fdbe530b" xlink:to="loc_us-gaap_LineOfCreditMember_01a0cb90-d253-77b9-84a9-c5c9db791ce5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeAxis_01a0cb90-d253-7d36-ad6c-57af2534165c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-76e6-b96e-538b3b8098ce" xlink:to="loc_us-gaap_MeasurementInputTypeAxis_01a0cb90-d253-7d36-ad6c-57af2534165c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7d36-ad6c-57af2534165c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_01a0cb90-d253-7d36-ad6c-57af2534165c" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7d36-ad6c-57af2534165c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7b3b-af91-9ac04336de6d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MeasurementInputTypeAxis_01a0cb90-d253-7d36-ad6c-57af2534165c" xlink:to="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7b3b-af91-9ac04336de6d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedTermMember_01a0cb90-d253-7235-9126-394af1708580" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputExpectedTermMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7b3b-af91-9ac04336de6d" xlink:to="loc_us-gaap_MeasurementInputExpectedTermMember_01a0cb90-d253-7235-9126-394af1708580" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputPriceVolatilityMember_01a0cb90-d253-7e0a-abf9-3c25f1e38da6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputPriceVolatilityMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7b3b-af91-9ac04336de6d" xlink:to="loc_us-gaap_MeasurementInputPriceVolatilityMember_01a0cb90-d253-7e0a-abf9-3c25f1e38da6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_01a0cb90-d253-727a-9e02-50a2cfd55d4f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputRiskFreeInterestRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7b3b-af91-9ac04336de6d" xlink:to="loc_us-gaap_MeasurementInputRiskFreeInterestRateMember_01a0cb90-d253-727a-9e02-50a2cfd55d4f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MeasurementInputExpectedDividendRateMember_01a0cb90-d253-74d4-bb35-db863d14ede9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_MeasurementInputExpectedDividendRateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MeasurementInputTypeDomain_01a0cb90-d253-7b3b-af91-9ac04336de6d" xlink:to="loc_us-gaap_MeasurementInputExpectedDividendRateMember_01a0cb90-d253-74d4-bb35-db863d14ede9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_01a0cb90-d253-7a05-ad91-edd08b4ff068" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_01a0cb90-d253-7a05-ad91-edd08b4ff068" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DividendsCommonStock_01a0cb90-d253-7a81-9bed-440903a94bf7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_DividendsCommonStock"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_DividendsCommonStock_01a0cb90-d253-7a81-9bed-440903a94bf7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_NumberOfVotesForEachShareOfCommonStockHeld_01a0cb90-d253-73e3-9673-ce4112c6bb8d" xlink:href="lung-20260930.xsd#lung_NumberOfVotesForEachShareOfCommonStockHeld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_lung_NumberOfVotesForEachShareOfCommonStockHeld_01a0cb90-d253-73e3-9673-ce4112c6bb8d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_01a0cb90-d253-79b8-af51-4847a89dbcb8" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_01a0cb90-d253-79b8-af51-4847a89dbcb8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_01a0cb90-d253-7aa1-b75c-bcd742ede923" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_01a0cb90-d253-7aa1-b75c-bcd742ede923" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue_01a0cb90-d253-7ce3-b211-363c7c953cb9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestExercisableAggregateIntrinsicValue_01a0cb90-d253-7ce3-b211-363c7c953cb9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_01a0cb90-d253-7624-861a-4b76ae9fafce" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_01a0cb90-d253-7624-861a-4b76ae9fafce" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_01a0cb90-d253-7b71-a548-fafb043c3b61" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_01a0cb90-d253-7b71-a548-fafb043c3b61" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_01a0cb90-d253-722f-8e62-473c5a35c9b6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_01a0cb90-d253-722f-8e62-473c5a35c9b6" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_01a0cb90-d253-75c6-9198-4f5fa45a2f5b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_01a0cb90-d253-75c6-9198-4f5fa45a2f5b" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_01a0cb90-d253-7fcf-808a-63c18b624a8c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_01a0cb90-d253-7fcf-808a-63c18b624a8c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharePrice_01a0cb90-d253-7519-95ba-5291b4531095" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_SharePrice_01a0cb90-d253-7519-95ba-5291b4531095" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_ShareBasedCompensationArrangementByShareBasedPaymentAwardTradingPeriod_01a0cb90-d253-7b6a-94d8-574ef8b27875" xlink:href="lung-20260930.xsd#lung_ShareBasedCompensationArrangementByShareBasedPaymentAwardTradingPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_lung_ShareBasedCompensationArrangementByShareBasedPaymentAwardTradingPeriod_01a0cb90-d253-7b6a-94d8-574ef8b27875" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_01a0cb90-d253-73ae-a175-d437c0dd9bc6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_01a0cb90-d253-73ae-a175-d437c0dd9bc6" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_01a0cb90-d253-7e43-bd39-5ecdac910c9d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_01a0cb90-d253-7e43-bd39-5ecdac910c9d" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_01a0cb90-d253-7434-8002-0f01f302ba8b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_01a0cb90-d253-7434-8002-0f01f302ba8b" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCostOfEquity_01a0cb90-d253-76ee-84a3-465e0377394b" xlink:href="lung-20260930.xsd#lung_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCostOfEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_lung_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsCostOfEquity_01a0cb90-d253-76ee-84a3-465e0377394b" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_01a0cb90-d253-7cc5-9d20-bf1f98c8aedd" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardRequisiteServicePeriod1_01a0cb90-d253-7cc5-9d20-bf1f98c8aedd" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_01a0cb90-d253-7243-98d5-b3edc389ae31" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsCapitalizedAmount_01a0cb90-d253-7243-98d5-b3edc389ae31" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_01a0cb90-d253-734b-bd68-e18d332174c2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_01a0cb90-d253-734b-bd68-e18d332174c2" xlink:type="arc" order="19"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_01a0cb90-d253-7140-b325-0939cae3e69e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_01a0cb90-d253-7140-b325-0939cae3e69e" xlink:type="arc" order="20"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_01a0cb90-d253-7513-a4c6-b112574eb746" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedShareBasedAwardsOtherThanOptions_01a0cb90-d253-7513-a4c6-b112574eb746" xlink:type="arc" order="21"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_01a0cb90-d253-7566-9331-44736798a377" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ClassOfWarrantOrRightNumberOfSecuritiesCalledByWarrantsOrRights_01a0cb90-d253-7566-9331-44736798a377" xlink:type="arc" order="22"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_01a0cb90-d253-7d09-831d-b15d587adc02" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_ClassOfWarrantOrRightExercisePriceOfWarrantsOrRights1_01a0cb90-d253-7d09-831d-b15d587adc02" xlink:type="arc" order="23"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstanding_01a0cb90-d253-74dd-abb2-98452fefcc6f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_WarrantsAndRightsOutstanding_01a0cb90-d253-74dd-abb2-98452fefcc6f" xlink:type="arc" order="24"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingTerm_01a0cb90-d253-7b1c-95e8-90d9bc5d8f64" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstandingTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingTerm_01a0cb90-d253-7b1c-95e8-90d9bc5d8f64" xlink:type="arc" order="25"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_01a0cb90-d253-71a2-825e-4708e273490e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantsAndRightsOutstandingMeasurementInput"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7d84-9060-a5ae6262c7ac" xlink:to="loc_us-gaap_WarrantsAndRightsOutstandingMeasurementInput_01a0cb90-d253-71a2-825e-4708e273490e" xlink:type="arc" order="26"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/StockholdersEquityScheduleofSharesReservedforFutureIssuanceDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#StockholdersEquityScheduleofSharesReservedforFutureIssuanceDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/StockholdersEquityScheduleofSharesReservedforFutureIssuanceDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockLineItems_01a0cb90-d253-7069-ab64-9705532bc460" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ClassOfStockLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfStockByClassTable_01a0cb90-d253-78a0-b86a-a918daaf40af" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfStockByClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a0cb90-d253-7069-ab64-9705532bc460" xlink:to="loc_us-gaap_ScheduleOfStockByClassTable_01a0cb90-d253-78a0-b86a-a918daaf40af" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7a11-bace-11ef65c89a4d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfStockByClassTable_01a0cb90-d253-78a0-b86a-a918daaf40af" xlink:to="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7a11-bace-11ef65c89a4d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7a11-bace-11ef65c89a4d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7a11-bace-11ef65c89a4d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7a11-bace-11ef65c89a4d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-78b9-bda3-2d65e146b74f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7a11-bace-11ef65c89a4d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-78b9-bda3-2d65e146b74f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_ShareBasedCompensationEmployeeStockOutstandingMember_01a0cb90-d253-7e66-b868-3513fb8b18cd" xlink:href="lung-20260930.xsd#lung_ShareBasedCompensationEmployeeStockOutstandingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-78b9-bda3-2d65e146b74f" xlink:to="loc_lung_ShareBasedCompensationEmployeeStockOutstandingMember_01a0cb90-d253-7e66-b868-3513fb8b18cd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_CommonStockRestrictedStockUnitsIssuedAndOutstandingMember_01a0cb90-d253-7fae-b660-088e746f4197" xlink:href="lung-20260930.xsd#lung_CommonStockRestrictedStockUnitsIssuedAndOutstandingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-78b9-bda3-2d65e146b74f" xlink:to="loc_lung_CommonStockRestrictedStockUnitsIssuedAndOutstandingMember_01a0cb90-d253-7fae-b660-088e746f4197" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_ShareBasedCompensationEmployeeStockAvailableForGrantMember_01a0cb90-d253-79d8-9972-6c0862b52000" xlink:href="lung-20260930.xsd#lung_ShareBasedCompensationEmployeeStockAvailableForGrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-78b9-bda3-2d65e146b74f" xlink:to="loc_lung_ShareBasedCompensationEmployeeStockAvailableForGrantMember_01a0cb90-d253-79d8-9972-6c0862b52000" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_01a0cb90-d253-7d83-9fdf-fb53d38ab7ae" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-78b9-bda3-2d65e146b74f" xlink:to="loc_us-gaap_EmployeeStockMember_01a0cb90-d253-7d83-9fdf-fb53d38ab7ae" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_01a0cb90-d253-750d-9f53-d632cc25f670" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-78b9-bda3-2d65e146b74f" xlink:to="loc_us-gaap_WarrantMember_01a0cb90-d253-750d-9f53-d632cc25f670" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_01a0cb90-d253-76b3-887f-949a2e4ce39e" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockLineItems_01a0cb90-d253-7069-ab64-9705532bc460" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_01a0cb90-d253-76b3-887f-949a2e4ce39e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/StockholdersEquityScheduleofPerformanceRestrictedStockUnitActivityDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#StockholdersEquityScheduleofPerformanceRestrictedStockUnitActivityDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/StockholdersEquityScheduleofPerformanceRestrictedStockUnitActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-70ff-8412-17a7886b92a9" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-7623-828d-eac41616a9a2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-70ff-8412-17a7886b92a9" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-7623-828d-eac41616a9a2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7a8c-bb66-39a11c168746" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-7623-828d-eac41616a9a2" xlink:to="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7a8c-bb66-39a11c168746" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7a8c-bb66-39a11c168746_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7a8c-bb66-39a11c168746" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7a8c-bb66-39a11c168746_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7a30-8d50-2f45278e7afa" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7a8c-bb66-39a11c168746" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7a30-8d50-2f45278e7afa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_01a0cb90-d253-79f1-974e-756f820e4986" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7a30-8d50-2f45278e7afa" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_01a0cb90-d253-79f1-974e-756f820e4986" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_01a0cb90-d253-7f24-b11e-bf0771be7816" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7a30-8d50-2f45278e7afa" xlink:to="loc_us-gaap_PerformanceSharesMember_01a0cb90-d253-7f24-b11e-bf0771be7816" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a0cb90-d253-743b-97db-4d19d037569a" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-70ff-8412-17a7886b92a9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a0cb90-d253-743b-97db-4d19d037569a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_01a0cb90-d253-779f-9847-1545d5682de0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a0cb90-d253-743b-97db-4d19d037569a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_01a0cb90-d253-779f-9847-1545d5682de0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_01a0cb90-d253-716e-8712-8c7a10a98bdc" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a0cb90-d253-743b-97db-4d19d037569a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_01a0cb90-d253-716e-8712-8c7a10a98bdc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_01a0cb90-d253-705a-86f6-8cc3e7650a19" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a0cb90-d253-743b-97db-4d19d037569a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_01a0cb90-d253-705a-86f6-8cc3e7650a19" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_01a0cb90-d253-72a3-aaf5-89bf44577ae7" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_01a0cb90-d253-743b-97db-4d19d037569a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_01a0cb90-d253-72a3-aaf5-89bf44577ae7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_01a0cb90-d253-7b13-b7c9-fa1077cc65a6" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a0cb90-d253-799b-ba4b-350803e700ca" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-70ff-8412-17a7886b92a9" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a0cb90-d253-799b-ba4b-350803e700ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_01a0cb90-d253-7e9f-9551-d71fc19cf81c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a0cb90-d253-799b-ba4b-350803e700ca" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_01a0cb90-d253-7e9f-9551-d71fc19cf81c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_01a0cb90-d253-7cb5-bbda-9a9ec52baff5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a0cb90-d253-799b-ba4b-350803e700ca" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_01a0cb90-d253-7cb5-bbda-9a9ec52baff5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_01a0cb90-d253-781c-b992-9e61b1464672" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a0cb90-d253-799b-ba4b-350803e700ca" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_01a0cb90-d253-781c-b992-9e61b1464672" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_01a0cb90-d253-7c04-8a90-b19bf70f6aa5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_01a0cb90-d253-799b-ba4b-350803e700ca" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_01a0cb90-d253-7c04-8a90-b19bf70f6aa5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_01a0cb90-d253-705c-8483-a7bcec2c6b16" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/StockholdersEquityScheduleofTotalStockBasedCompensationDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#StockholdersEquityScheduleofTotalStockBasedCompensationDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/StockholdersEquityScheduleofTotalStockBasedCompensationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7fd4-b55c-2573281224be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-7e13-aba5-e0dcbd1facb0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7fd4-b55c-2573281224be" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-7e13-aba5-e0dcbd1facb0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_01a0cb90-d253-7844-ac5c-6ac9e63d8a69" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_StatementOfIncomeLocationBalanceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-7e13-aba5-e0dcbd1facb0" xlink:to="loc_us-gaap_StatementOfIncomeLocationBalanceAxis_01a0cb90-d253-7844-ac5c-6ac9e63d8a69" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7d2d-8ff4-3b7acf470015" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-7e13-aba5-e0dcbd1facb0" xlink:to="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7d2d-8ff4-3b7acf470015" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7d2d-8ff4-3b7acf470015_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7d2d-8ff4-3b7acf470015" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7d2d-8ff4-3b7acf470015_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7acf-b308-ff8ebc14a147" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7d2d-8ff4-3b7acf470015" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7acf-b308-ff8ebc14a147" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_StockOptionsAndRestrictedStockUnitsMember_01a0cb90-d253-72bd-bb0a-e1074f8c00c3" xlink:href="lung-20260930.xsd#lung_StockOptionsAndRestrictedStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7acf-b308-ff8ebc14a147" xlink:to="loc_lung_StockOptionsAndRestrictedStockUnitsMember_01a0cb90-d253-72bd-bb0a-e1074f8c00c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_01a0cb90-d253-74b8-9cdb-cbc455d1b95c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7acf-b308-ff8ebc14a147" xlink:to="loc_us-gaap_EmployeeStockMember_01a0cb90-d253-74b8-9cdb-cbc455d1b95c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a0cb90-d253-7402-9e75-16e52e417b13" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7fd4-b55c-2573281224be" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a0cb90-d253-7402-9e75-16e52e417b13" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/StockholdersEquityScheduleofTotalStockBasedCompensationDetails_1" xlink:type="simple" xlink:href="lung-20260930.xsd#StockholdersEquityScheduleofTotalStockBasedCompensationDetails_1"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/StockholdersEquityScheduleofTotalStockBasedCompensationDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7fd4-b55c-2573281224be" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-7e13-aba5-e0dcbd1facb0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7fd4-b55c-2573281224be" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-7e13-aba5-e0dcbd1facb0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7d2d-8ff4-3b7acf470015" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_01a0cb90-d253-7e13-aba5-e0dcbd1facb0" xlink:to="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7d2d-8ff4-3b7acf470015" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7d2d-8ff4-3b7acf470015_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7d2d-8ff4-3b7acf470015" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7d2d-8ff4-3b7acf470015_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7acf-b308-ff8ebc14a147" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_01a0cb90-d253-7d2d-8ff4-3b7acf470015" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7acf-b308-ff8ebc14a147" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_StockOptionsAndRestrictedStockUnitsMember_01a0cb90-d253-72bd-bb0a-e1074f8c00c3" xlink:href="lung-20260930.xsd#lung_StockOptionsAndRestrictedStockUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7acf-b308-ff8ebc14a147" xlink:to="loc_lung_StockOptionsAndRestrictedStockUnitsMember_01a0cb90-d253-72bd-bb0a-e1074f8c00c3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_01a0cb90-d253-74b8-9cdb-cbc455d1b95c" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_01a0cb90-d253-7acf-b308-ff8ebc14a147" xlink:to="loc_us-gaap_EmployeeStockMember_01a0cb90-d253-74b8-9cdb-cbc455d1b95c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a0cb90-d253-7402-9e75-16e52e417b13" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_01a0cb90-d253-7fd4-b55c-2573281224be" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_01a0cb90-d253-7402-9e75-16e52e417b13" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/NetLossperShareAttributabletoCommonStockholdersScheduleofExcludedPotentiallyDilutiveSecuritiesOutstandingDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#NetLossperShareAttributabletoCommonStockholdersScheduleofExcludedPotentiallyDilutiveSecuritiesOutstandingDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/NetLossperShareAttributabletoCommonStockholdersScheduleofExcludedPotentiallyDilutiveSecuritiesOutstandingDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_01a0cb90-d253-7e0f-948a-047798816084" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_01a0cb90-d253-79b0-9004-7a3cb09b2ca2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_01a0cb90-d253-7e0f-948a-047798816084" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_01a0cb90-d253-79b0-9004-7a3cb09b2ca2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_01a0cb90-d253-7d00-b330-8d3997e8ed3f" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_01a0cb90-d253-79b0-9004-7a3cb09b2ca2" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_01a0cb90-d253-7d00-b330-8d3997e8ed3f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0cb90-d253-7d00-b330-8d3997e8ed3f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_01a0cb90-d253-7d00-b330-8d3997e8ed3f" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0cb90-d253-7d00-b330-8d3997e8ed3f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0cb90-d253-7968-9c3d-51023cd2343b" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_01a0cb90-d253-7d00-b330-8d3997e8ed3f" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0cb90-d253-7968-9c3d-51023cd2343b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_01a0cb90-d253-794f-a6e4-6139b7e71ac0" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0cb90-d253-7968-9c3d-51023cd2343b" xlink:to="loc_us-gaap_EmployeeStockOptionMember_01a0cb90-d253-794f-a6e4-6139b7e71ac0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockMember_01a0cb90-d253-76a3-b5cd-cd622eb63b0d" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RestrictedStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0cb90-d253-7968-9c3d-51023cd2343b" xlink:to="loc_us-gaap_RestrictedStockMember_01a0cb90-d253-76a3-b5cd-cd622eb63b0d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WarrantMember_01a0cb90-d253-748c-873a-5f1a252d65b3" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_WarrantMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0cb90-d253-7968-9c3d-51023cd2343b" xlink:to="loc_us-gaap_WarrantMember_01a0cb90-d253-748c-873a-5f1a252d65b3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_01a0cb90-d253-7072-89fc-31cdc630b2b2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_01a0cb90-d253-7968-9c3d-51023cd2343b" xlink:to="loc_us-gaap_EmployeeStockMember_01a0cb90-d253-7072-89fc-31cdc630b2b2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_01a0cb90-d253-76c9-b616-43beac4a9311" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_01a0cb90-d253-7e0f-948a-047798816084" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_01a0cb90-d253-76c9-b616-43beac4a9311" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/SegmentInformationScheduleofRevenuebyGeographicAreaDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#SegmentInformationScheduleofRevenuebyGeographicAreaDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/SegmentInformationScheduleofRevenuebyGeographicAreaDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_01a0cb90-d253-7f13-95da-eb0a049e69c4" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_01a0cb90-d253-7a31-9758-4ea5b5661b32" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_01a0cb90-d253-7f13-95da-eb0a049e69c4" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_01a0cb90-d253-7a31-9758-4ea5b5661b32" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_01a0cb90-d253-7ee8-9c19-03c1472a1965" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_01a0cb90-d253-7a31-9758-4ea5b5661b32" xlink:to="loc_srt_StatementGeographicalAxis_01a0cb90-d253-7ee8-9c19-03c1472a1965" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7ee8-9c19-03c1472a1965_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_01a0cb90-d253-7ee8-9c19-03c1472a1965" xlink:to="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7ee8-9c19-03c1472a1965_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-71f7-be83-af7346e105a4" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_01a0cb90-d253-7ee8-9c19-03c1472a1965" xlink:to="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-71f7-be83-af7346e105a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_01a0cb90-d253-7edc-8c8b-f76d2e2c5316" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-71f7-be83-af7346e105a4" xlink:to="loc_country_US_01a0cb90-d253-7edc-8c8b-f76d2e2c5316" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EMEAMember_01a0cb90-d253-7663-9dc3-3912e81d2fd5" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EMEAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-71f7-be83-af7346e105a4" xlink:to="loc_us-gaap_EMEAMember_01a0cb90-d253-7663-9dc3-3912e81d2fd5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_AsiaPacificMember_01a0cb90-d253-773d-81ff-9b3e74841b93" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_AsiaPacificMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-71f7-be83-af7346e105a4" xlink:to="loc_srt_AsiaPacificMember_01a0cb90-d253-773d-81ff-9b3e74841b93" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_OtherInternationalGeographicalAreasMember_01a0cb90-d253-7023-99db-e0c817c4ab8c" xlink:href="lung-20260930.xsd#lung_OtherInternationalGeographicalAreasMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-71f7-be83-af7346e105a4" xlink:to="loc_lung_OtherInternationalGeographicalAreasMember_01a0cb90-d253-7023-99db-e0c817c4ab8c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_01a0cb90-d253-7095-80ab-7cb5e13a83cf" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_01a0cb90-d253-7f13-95da-eb0a049e69c4" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_01a0cb90-d253-7095-80ab-7cb5e13a83cf" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.pulmonx.com/role/SegmentInformationScheduleofLongLivedAssetsbyGeographicalAreaDetails" xlink:type="simple" xlink:href="lung-20260930.xsd#SegmentInformationScheduleofLongLivedAssetsbyGeographicalAreaDetails"/>
  <link:definitionLink xlink:role="http://www.pulmonx.com/role/SegmentInformationScheduleofLongLivedAssetsbyGeographicalAreaDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_01a0cb90-d253-7fe6-89ad-d046578caaa2" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_01a0cb90-d253-7332-bd1b-735d98f64802" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_01a0cb90-d253-7fe6-89ad-d046578caaa2" xlink:to="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_01a0cb90-d253-7332-bd1b-735d98f64802" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_01a0cb90-d253-7d92-9e5f-76fcf1375c9d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRevenuesFromExternalCustomersAndLongLivedAssetsTable_01a0cb90-d253-7332-bd1b-735d98f64802" xlink:to="loc_srt_StatementGeographicalAxis_01a0cb90-d253-7d92-9e5f-76fcf1375c9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7d92-9e5f-76fcf1375c9d_default" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_01a0cb90-d253-7d92-9e5f-76fcf1375c9d" xlink:to="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7d92-9e5f-76fcf1375c9d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7e33-bfaf-44d1ca3bbbf6" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_01a0cb90-d253-7d92-9e5f-76fcf1375c9d" xlink:to="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7e33-bfaf-44d1ca3bbbf6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_01a0cb90-d253-7671-b895-0025db9498a4" xlink:href="https://xbrl.sec.gov/country/2026/country-2026.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7e33-bfaf-44d1ca3bbbf6" xlink:to="loc_country_US_01a0cb90-d253-7671-b895-0025db9498a4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EMEAMember_01a0cb90-d253-7c41-b839-399a8a65bf38" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_EMEAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7e33-bfaf-44d1ca3bbbf6" xlink:to="loc_us-gaap_EMEAMember_01a0cb90-d253-7c41-b839-399a8a65bf38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_AsiaPacificMember_01a0cb90-d253-7821-9fdd-8118f3fdad7d" xlink:href="https://xbrl.fasb.org/srt/2026/elts/srt-2026.xsd#srt_AsiaPacificMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_01a0cb90-d253-7e33-bfaf-44d1ca3bbbf6" xlink:to="loc_srt_AsiaPacificMember_01a0cb90-d253-7821-9fdd-8118f3fdad7d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncurrentAssets_01a0cb90-d253-7b80-9d1d-3f359bb00510" xlink:href="https://xbrl.fasb.org/us-gaap/2026/elts/us-gaap-2026.xsd#us-gaap_NoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesFromExternalCustomersAndLongLivedAssetsLineItems_01a0cb90-d253-7fe6-89ad-d046578caaa2" xlink:to="loc_us-gaap_NoncurrentAssets_01a0cb90-d253-7b80-9d1d-3f359bb00510" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2026/ecd-2026.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_RichFerrariMember_01a177d7-9a60-7d2c-bc4f-d677f8740891" xlink:href="lung-20260930.xsd#lung_RichFerrariMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_lung_RichFerrariMember_01a177d7-9a60-7d2c-bc4f-d677f8740891" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_lung_BeranRoseMember_01a177d8-a74f-7b3a-8235-7f6df6f12693" xlink:href="lung-20260930.xsd#lung_BeranRoseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_lung_BeranRoseMember_01a177d8-a74f-7b3a-8235-7f6df6f12693" xlink:type="arc" order="3"/>
  </link:definitionLink>
</link:linkbase>
