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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019c7730-46e9-7212-bc71-75c229f2667a,g:9ff11a8a-3051-4cda-8828-c63f363e56da-->
<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.geron.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="simple" xlink:href="gern-20260331.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS"/>
  <link:definitionLink xlink:role="http://www.geron.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFOPERATIONS" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019c7730-47b6-78eb-907d-f40c0cfcb66e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:to="loc_us-gaap_StatementTable_019c7730-47b6-78eb-907d-f40c0cfcb66e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019c7730-47b6-7b1f-b629-1321b325e9b9" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019c7730-47b6-78eb-907d-f40c0cfcb66e" xlink:to="loc_srt_ProductOrServiceAxis_019c7730-47b6-7b1f-b629-1321b325e9b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c7730-47b6-7b1f-b629-1321b325e9b9_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019c7730-47b6-7b1f-b629-1321b325e9b9" xlink:to="loc_srt_ProductsAndServicesDomain_019c7730-47b6-7b1f-b629-1321b325e9b9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c7730-47b6-7ee5-a30b-0fa9c0c04c90" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019c7730-47b6-7b1f-b629-1321b325e9b9" xlink:to="loc_srt_ProductsAndServicesDomain_019c7730-47b6-7ee5-a30b-0fa9c0c04c90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductMember_019c7730-47b6-76a9-9f24-513bc1e9a7fc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c7730-47b6-7ee5-a30b-0fa9c0c04c90" xlink:to="loc_us-gaap_ProductMember_019c7730-47b6-76a9-9f24-513bc1e9a7fc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RoyaltyMember_019c7730-47b6-7af1-b64d-4524584c1c39" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RoyaltyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c7730-47b6-7ee5-a30b-0fa9c0c04c90" xlink:to="loc_us-gaap_RoyaltyMember_019c7730-47b6-7af1-b64d-4524584c1c39" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesAbstract_019c7730-47b6-768e-92ba-b4563a85b3c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:to="loc_us-gaap_RevenuesAbstract_019c7730-47b6-768e-92ba-b4563a85b3c6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c7730-47b6-708d-9e40-d256ee0dcb88" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_019c7730-47b6-768e-92ba-b4563a85b3c6" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c7730-47b6-708d-9e40-d256ee0dcb88" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpensesAbstract_019c7730-47b6-7d68-8102-2eda0301f8ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:to="loc_us-gaap_OperatingExpensesAbstract_019c7730-47b6-7d68-8102-2eda0301f8ae" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingCostsAndExpenses_019c7730-47b6-7022-8fed-912b7aafa882" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingCostsAndExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c7730-47b6-7d68-8102-2eda0301f8ae" xlink:to="loc_us-gaap_OperatingCostsAndExpenses_019c7730-47b6-7022-8fed-912b7aafa882" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost_019c7730-47b6-758d-a18a-d1611da7e997" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c7730-47b6-7d68-8102-2eda0301f8ae" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost_019c7730-47b6-758d-a18a-d1611da7e997" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_019c7730-47b6-73e0-ab67-1133fc31c277" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c7730-47b6-7d68-8102-2eda0301f8ae" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_019c7730-47b6-73e0-ab67-1133fc31c277" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_019dbffb-33c0-782d-9e28-567d3eab5590" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c7730-47b6-7d68-8102-2eda0301f8ae" xlink:to="loc_us-gaap_RestructuringCharges_019dbffb-33c0-782d-9e28-567d3eab5590" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_019c7730-47b6-7239-bdf3-018172606dcb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c7730-47b6-7d68-8102-2eda0301f8ae" xlink:to="loc_us-gaap_OperatingExpenses_019c7730-47b6-7239-bdf3-018172606dcb" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_019c7730-47b6-78ba-b96a-d421d0b55ac5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:to="loc_us-gaap_OperatingIncomeLoss_019c7730-47b6-78ba-b96a-d421d0b55ac5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeInterest_019c7730-47b6-7fe0-8ddf-ac84aba4d39e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:to="loc_us-gaap_InvestmentIncomeInterest_019c7730-47b6-7fe0-8ddf-ac84aba4d39e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseNonoperating_019c7730-47b6-7eff-96f0-3e467eb190d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:to="loc_us-gaap_InterestExpenseNonoperating_019c7730-47b6-7eff-96f0-3e467eb190d4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019c7730-47b6-7730-bdff-7e04b5869c81" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019c7730-47b6-7730-bdff-7e04b5869c81" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019c7730-47b6-7bc9-b911-cc6e0ecbb27b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:to="loc_us-gaap_NetIncomeLoss_019c7730-47b6-7bc9-b911-cc6e0ecbb27b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_019c7730-47b6-7df5-a53b-dfef7ebd58ea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:to="loc_us-gaap_EarningsPerShareBasic_019c7730-47b6-7df5-a53b-dfef7ebd58ea" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_019c7730-47b6-7668-b575-ca07b20d9ed1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:to="loc_us-gaap_EarningsPerShareDiluted_019c7730-47b6-7668-b575-ca07b20d9ed1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019c7730-47b6-752a-91bb-32bf8353a7ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019c7730-47b6-752a-91bb-32bf8353a7ce" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019c7730-47b6-7650-8523-dea51cbc539f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-7be0-94fa-10cb86d6df58" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019c7730-47b6-7650-8523-dea51cbc539f" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.geron.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="simple" xlink:href="gern-20260331.xsd#CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY"/>
  <link:definitionLink xlink:role="http://www.geron.com/role/CONDENSEDCONSOLIDATEDSTATEMENTSOFSTOCKHOLDERSEQUITY" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019c7730-47b6-70ae-ab10-07119e1ed646" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019c7730-47b6-728a-9de0-26d60ce4d29b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-70ae-ab10-07119e1ed646" xlink:to="loc_us-gaap_StatementTable_019c7730-47b6-728a-9de0-26d60ce4d29b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019c7730-47b6-7557-98aa-6d3d93eb5dc4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019c7730-47b6-728a-9de0-26d60ce4d29b" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019c7730-47b6-7557-98aa-6d3d93eb5dc4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019c7730-47b6-7557-98aa-6d3d93eb5dc4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019c7730-47b6-7557-98aa-6d3d93eb5dc4" xlink:to="loc_us-gaap_EquityComponentDomain_019c7730-47b6-7557-98aa-6d3d93eb5dc4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019c7730-47b6-7b91-accf-5eed2b21550d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019c7730-47b6-7557-98aa-6d3d93eb5dc4" xlink:to="loc_us-gaap_EquityComponentDomain_019c7730-47b6-7b91-accf-5eed2b21550d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019c7730-47b6-77ab-bda8-2adb04e8a93d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c7730-47b6-7b91-accf-5eed2b21550d" xlink:to="loc_us-gaap_CommonStockMember_019c7730-47b6-77ab-bda8-2adb04e8a93d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019c7730-47b6-77cf-ae1c-624ddb276295" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c7730-47b6-7b91-accf-5eed2b21550d" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019c7730-47b6-77cf-ae1c-624ddb276295" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019c7730-47b6-7f31-84b5-580efd89414f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c7730-47b6-7b91-accf-5eed2b21550d" xlink:to="loc_us-gaap_RetainedEarningsMember_019c7730-47b6-7f31-84b5-580efd89414f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019c7730-47b6-7844-b9fb-59c685dae605" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c7730-47b6-7b91-accf-5eed2b21550d" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019c7730-47b6-7844-b9fb-59c685dae605" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c7730-47b6-70ae-ab10-07119e1ed646" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019c7730-47b6-771c-9c24-1a21e53f227a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019c7730-47b6-771c-9c24-1a21e53f227a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019c7730-47b6-74dc-919a-5ffd74b8366b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_us-gaap_StockholdersEquity_019c7730-47b6-74dc-919a-5ffd74b8366b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019c7730-47b6-712f-b69b-1877efafac39" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_us-gaap_NetIncomeLoss_019c7730-47b6-712f-b69b-1877efafac39" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentBeforeReclassificationAdjustmentsNetOfTax_019c7730-47b6-7160-9b6b-57e1b9f231a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentBeforeReclassificationAdjustmentsNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossAvailableForSaleSecuritiesAdjustmentBeforeReclassificationAdjustmentsNetOfTax_019c7730-47b6-7160-9b6b-57e1b9f231a0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_019c7730-47b6-73aa-b71f-c26c1a48e0fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_019c7730-47b6-73aa-b71f-c26c1a48e0fa" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_StockIssuedDuringPeriodSharesExerciseOfPrefundedWarrant_019dd608-eb9e-7e3e-82d3-03a3bc127101" xlink:href="gern-20260331.xsd#gern_StockIssuedDuringPeriodSharesExerciseOfPrefundedWarrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_gern_StockIssuedDuringPeriodSharesExerciseOfPrefundedWarrant_019dd608-eb9e-7e3e-82d3-03a3bc127101" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_StockIssuedDuringPeriodValueExerciseOfPrefundedWarrant_019dd608-eb9e-7e08-a3a6-61aa1eb34a10" xlink:href="gern-20260331.xsd#gern_StockIssuedDuringPeriodValueExerciseOfPrefundedWarrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_gern_StockIssuedDuringPeriodValueExerciseOfPrefundedWarrant_019dd608-eb9e-7e08-a3a6-61aa1eb34a10" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_IssuanceOfCommonStockInConnectionWithWarrantExerciseValue_019c7730-47b6-7835-8b8d-a07a5a452679" xlink:href="gern-20260331.xsd#gern_IssuanceOfCommonStockInConnectionWithWarrantExerciseValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_gern_IssuanceOfCommonStockInConnectionWithWarrantExerciseValue_019c7730-47b6-7835-8b8d-a07a5a452679" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019dc238-fc4e-753c-90df-f9693539671f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019dc238-fc4e-753c-90df-f9693539671f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019c7730-47b6-7b41-b853-519f95e2b6b6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_us-gaap_StockIssuedDuringPeriodValueShareBasedCompensation_019c7730-47b6-7b41-b853-519f95e2b6b6" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_APICShareBasedPaymentArrangementOptionIncreaseForCostRecognitionShares_019dc256-615f-76db-a552-b2ad24e57a7a" xlink:href="gern-20260331.xsd#gern_APICShareBasedPaymentArrangementOptionIncreaseForCostRecognitionShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_gern_APICShareBasedPaymentArrangementOptionIncreaseForCostRecognitionShares_019dc256-615f-76db-a552-b2ad24e57a7a" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition_019df88d-ce1e-7632-92f5-1ced2fa7bbb5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationStockOptionsRequisiteServicePeriodRecognition_019df88d-ce1e-7632-92f5-1ced2fa7bbb5" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationEmployeeStockPurchaseProgramRequisiteServicePeriodRecognition_019df88e-008a-73c5-bade-73a208144c94" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationEmployeeStockPurchaseProgramRequisiteServicePeriodRecognition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c7730-47b6-7c46-949a-c3a9d0c529a2" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalShareBasedCompensationEmployeeStockPurchaseProgramRequisiteServicePeriodRecognition_019df88e-008a-73c5-bade-73a208144c94" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019c7730-47b6-7858-9c19-b1dacd4dddd1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019c7730-47b6-7269-9db8-6d2263d2c8fd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.geron.com/role/REVENUERECOGNITIONDetails" xlink:type="simple" xlink:href="gern-20260331.xsd#REVENUERECOGNITIONDetails"/>
  <link:definitionLink xlink:role="http://www.geron.com/role/REVENUERECOGNITIONDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019c7730-4892-7c72-9393-95a735cac451" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019c7730-4892-7364-9a9b-d4cfa1b988cb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019c7730-4892-7c72-9393-95a735cac451" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019c7730-4892-7364-9a9b-d4cfa1b988cb" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019c7730-4892-7428-8801-300020f3e936" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019c7730-4892-7364-9a9b-d4cfa1b988cb" xlink:to="loc_srt_ProductOrServiceAxis_019c7730-4892-7428-8801-300020f3e936" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c7730-4892-7428-8801-300020f3e936_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019c7730-4892-7428-8801-300020f3e936" xlink:to="loc_srt_ProductsAndServicesDomain_019c7730-4892-7428-8801-300020f3e936_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c7730-4892-781c-9910-405a5a2bdece" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019c7730-4892-7428-8801-300020f3e936" xlink:to="loc_srt_ProductsAndServicesDomain_019c7730-4892-781c-9910-405a5a2bdece" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductMember_019c7730-4892-7b5e-83ec-8f931fba3b2e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c7730-4892-781c-9910-405a5a2bdece" xlink:to="loc_us-gaap_ProductMember_019c7730-4892-7b5e-83ec-8f931fba3b2e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_RevenuesBeforeAdjustmentsMember_019c7730-4892-712a-91e6-1b750a031ebf" xlink:href="gern-20260331.xsd#gern_RevenuesBeforeAdjustmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductMember_019c7730-4892-7b5e-83ec-8f931fba3b2e" xlink:to="loc_gern_RevenuesBeforeAdjustmentsMember_019c7730-4892-712a-91e6-1b750a031ebf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_TotalGrossToNetAdjustmentsMember_019c7730-4892-755a-a93a-d26afebe95f1" xlink:href="gern-20260331.xsd#gern_TotalGrossToNetAdjustmentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ProductMember_019c7730-4892-7b5e-83ec-8f931fba3b2e" xlink:to="loc_gern_TotalGrossToNetAdjustmentsMember_019c7730-4892-755a-a93a-d26afebe95f1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_ChargebacksMember_019c7730-4892-7a20-94bb-c0fa96e8059f" xlink:href="gern-20260331.xsd#gern_ChargebacksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gern_TotalGrossToNetAdjustmentsMember_019c7730-4892-755a-a93a-d26afebe95f1" xlink:to="loc_gern_ChargebacksMember_019c7730-4892-7a20-94bb-c0fa96e8059f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_DistributorServiceFeesMember_019dc2a1-7101-75d9-8795-167c1d2c227f" xlink:href="gern-20260331.xsd#gern_DistributorServiceFeesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gern_TotalGrossToNetAdjustmentsMember_019c7730-4892-755a-a93a-d26afebe95f1" xlink:to="loc_gern_DistributorServiceFeesMember_019dc2a1-7101-75d9-8795-167c1d2c227f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_GovernmentRebatesMember_019c7730-4892-7ba0-8e57-9075329cb2c9" xlink:href="gern-20260331.xsd#gern_GovernmentRebatesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gern_TotalGrossToNetAdjustmentsMember_019c7730-4892-755a-a93a-d26afebe95f1" xlink:to="loc_gern_GovernmentRebatesMember_019c7730-4892-7ba0-8e57-9075329cb2c9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_SalesReturnsAndAllowanceMember_019c7730-4892-72d5-9c96-e01c61bafb7d" xlink:href="gern-20260331.xsd#gern_SalesReturnsAndAllowanceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gern_TotalGrossToNetAdjustmentsMember_019c7730-4892-755a-a93a-d26afebe95f1" xlink:to="loc_gern_SalesReturnsAndAllowanceMember_019c7730-4892-72d5-9c96-e01c61bafb7d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c7730-4892-766b-a00d-f33186e6c395" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019c7730-4892-7c72-9393-95a735cac451" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c7730-4892-766b-a00d-f33186e6c395" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.geron.com/role/FINANCIALINSTRUMENTSCashEquivalentsandMarketableSecuritiesDetails" xlink:type="simple" xlink:href="gern-20260331.xsd#FINANCIALINSTRUMENTSCashEquivalentsandMarketableSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://www.geron.com/role/FINANCIALINSTRUMENTSCashEquivalentsandMarketableSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SummaryOfInvestmentHoldingsLineItems_019dc2a8-10b3-7f9d-b261-10bd44147d08" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SummaryOfInvestmentHoldingsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SummaryOfInvestmentHoldingsTable_019dc2a8-10b3-7b57-8d23-809a88972d9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SummaryOfInvestmentHoldingsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SummaryOfInvestmentHoldingsLineItems_019dc2a8-10b3-7f9d-b261-10bd44147d08" xlink:to="loc_us-gaap_SummaryOfInvestmentHoldingsTable_019dc2a8-10b3-7b57-8d23-809a88972d9d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019dc2a8-10b3-7f71-9512-4c01394e66f9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SummaryOfInvestmentHoldingsTable_019dc2a8-10b3-7b57-8d23-809a88972d9d" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019dc2a8-10b3-7f71-9512-4c01394e66f9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7f71-9512-4c01394e66f9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019dc2a8-10b3-7f71-9512-4c01394e66f9" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7f71-9512-4c01394e66f9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-71ec-b209-6355fc959c28" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019dc2a8-10b3-7f71-9512-4c01394e66f9" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-71ec-b209-6355fc959c28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_019dc2a8-10b3-740b-9a4d-5b57674ad090" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-71ec-b209-6355fc959c28" xlink:to="loc_us-gaap_MoneyMarketFundsMember_019dc2a8-10b3-740b-9a4d-5b57674ad090" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019dc2a8-10b3-7460-9568-0e082d88005a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-71ec-b209-6355fc959c28" xlink:to="loc_us-gaap_CommercialPaperMember_019dc2a8-10b3-7460-9568-0e082d88005a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_019dc2a8-10b3-7bfa-bf5d-0aff040c3e84" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CertificatesOfDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-71ec-b209-6355fc959c28" xlink:to="loc_us-gaap_CertificatesOfDepositMember_019dc2a8-10b3-7bfa-bf5d-0aff040c3e84" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_USTreasurySecuritiesDueInLessThanOneYearMember_019dc2a8-10b3-7150-9847-682a34abe218" xlink:href="gern-20260331.xsd#gern_USTreasurySecuritiesDueInLessThanOneYearMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-71ec-b209-6355fc959c28" xlink:to="loc_gern_USTreasurySecuritiesDueInLessThanOneYearMember_019dc2a8-10b3-7150-9847-682a34abe218" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_MunicipalSecuritiesDueInLessThanOneYearMember_019dc2a8-10b3-70b1-80c4-8c36ed935ae2" xlink:href="gern-20260331.xsd#gern_MunicipalSecuritiesDueInLessThanOneYearMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-71ec-b209-6355fc959c28" xlink:to="loc_gern_MunicipalSecuritiesDueInLessThanOneYearMember_019dc2a8-10b3-70b1-80c4-8c36ed935ae2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_MunicipalSecuritiesDueInOneToTwoYearsMember_019df09a-db2b-7cf6-8ae3-649ee36d8a3b" xlink:href="gern-20260331.xsd#gern_MunicipalSecuritiesDueInOneToTwoYearsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-71ec-b209-6355fc959c28" xlink:to="loc_gern_MunicipalSecuritiesDueInOneToTwoYearsMember_019df09a-db2b-7cf6-8ae3-649ee36d8a3b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember_019dc2a8-10b3-78dd-a4c7-896ee483496a" xlink:href="gern-20260331.xsd#gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-71ec-b209-6355fc959c28" xlink:to="loc_gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember_019dc2a8-10b3-78dd-a4c7-896ee483496a" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_CommercialPaperDueInLessThanOneYearMember_019dc2a8-10b3-7339-9b35-3fb9d8c5e6fb" xlink:href="gern-20260331.xsd#gern_CommercialPaperDueInLessThanOneYearMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-71ec-b209-6355fc959c28" xlink:to="loc_gern_CommercialPaperDueInLessThanOneYearMember_019dc2a8-10b3-7339-9b35-3fb9d8c5e6fb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_CorporateNoteSecuritiesDueInLessThanOneYearMember_019dc2a8-10b3-7fa2-a7ff-f216b3f0c3dc" xlink:href="gern-20260331.xsd#gern_CorporateNoteSecuritiesDueInLessThanOneYearMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-71ec-b209-6355fc959c28" xlink:to="loc_gern_CorporateNoteSecuritiesDueInLessThanOneYearMember_019dc2a8-10b3-7fa2-a7ff-f216b3f0c3dc" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember_019dc2a8-10b3-7ffb-8089-c0c87b50fc2d" xlink:href="gern-20260331.xsd#gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-71ec-b209-6355fc959c28" xlink:to="loc_gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember_019dc2a8-10b3-7ffb-8089-c0c87b50fc2d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAbstract_019dc2a8-10b3-7e41-b531-6caa9aa752a2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SummaryOfInvestmentHoldingsLineItems_019dc2a8-10b3-7f9d-b261-10bd44147d08" xlink:to="loc_us-gaap_CashAndCashEquivalentsAbstract_019dc2a8-10b3-7e41-b531-6caa9aa752a2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashEquivalentsAtCarryingValue_019dc2a8-10b3-7a0c-a835-cbd0d801f262" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsAbstract_019dc2a8-10b3-7e41-b531-6caa9aa752a2" xlink:to="loc_us-gaap_CashEquivalentsAtCarryingValue_019dc2a8-10b3-7a0c-a835-cbd0d801f262" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_CashAndCashEquivalentsUnrealizedGains_019dc2a8-10b3-7002-82e0-7d7d4991efb0" xlink:href="gern-20260331.xsd#gern_CashAndCashEquivalentsUnrealizedGains"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsAbstract_019dc2a8-10b3-7e41-b531-6caa9aa752a2" xlink:to="loc_gern_CashAndCashEquivalentsUnrealizedGains_019dc2a8-10b3-7002-82e0-7d7d4991efb0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_CashAndCashEquivalentsUnrealizedLosses_019dc2a8-10b3-7758-bfed-39194d70d16d" xlink:href="gern-20260331.xsd#gern_CashAndCashEquivalentsUnrealizedLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsAbstract_019dc2a8-10b3-7e41-b531-6caa9aa752a2" xlink:to="loc_gern_CashAndCashEquivalentsUnrealizedLosses_019dc2a8-10b3-7758-bfed-39194d70d16d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019dc2a8-10b3-745c-a1a9-465d24643cca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashAndCashEquivalentsAbstract_019dc2a8-10b3-7e41-b531-6caa9aa752a2" xlink:to="loc_us-gaap_CashAndCashEquivalentsFairValueDisclosure_019dc2a8-10b3-745c-a1a9-465d24643cca" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndInvestmentsAbstract_019dc2a8-10b3-7a42-9fd8-4460921d0a2f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndInvestmentsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SummaryOfInvestmentHoldingsLineItems_019dc2a8-10b3-7f9d-b261-10bd44147d08" xlink:to="loc_us-gaap_RestrictedCashAndInvestmentsAbstract_019dc2a8-10b3-7a42-9fd8-4460921d0a2f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue_019dc2a8-10b3-72e2-8e58-e33c85a4ecf2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndInvestmentsAbstract_019dc2a8-10b3-7a42-9fd8-4460921d0a2f" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsAtCarryingValue_019dc2a8-10b3-72e2-8e58-e33c85a4ecf2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_RestrictedCashAndCashEquivalentsGrossUnrealizedGains_019dc2a8-10b3-7b7e-baae-c01b6304be64" xlink:href="gern-20260331.xsd#gern_RestrictedCashAndCashEquivalentsGrossUnrealizedGains"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndInvestmentsAbstract_019dc2a8-10b3-7a42-9fd8-4460921d0a2f" xlink:to="loc_gern_RestrictedCashAndCashEquivalentsGrossUnrealizedGains_019dc2a8-10b3-7b7e-baae-c01b6304be64" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_RestrictedCashAndCashEquivalentsGrossUnrealizedLosses_019dc2a8-10b3-72d9-9673-f0a886ecd514" xlink:href="gern-20260331.xsd#gern_RestrictedCashAndCashEquivalentsGrossUnrealizedLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndInvestmentsAbstract_019dc2a8-10b3-7a42-9fd8-4460921d0a2f" xlink:to="loc_gern_RestrictedCashAndCashEquivalentsGrossUnrealizedLosses_019dc2a8-10b3-72d9-9673-f0a886ecd514" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedInvestmentsAtFairValue_019dc2a8-10b3-7245-91b4-548aec89269a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedInvestmentsAtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndInvestmentsAbstract_019dc2a8-10b3-7a42-9fd8-4460921d0a2f" xlink:to="loc_us-gaap_RestrictedInvestmentsAtFairValue_019dc2a8-10b3-7245-91b4-548aec89269a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesAbstract_019dc2a8-10b3-7abc-a123-569214012997" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SummaryOfInvestmentHoldingsLineItems_019dc2a8-10b3-7f9d-b261-10bd44147d08" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesAbstract_019dc2a8-10b3-7abc-a123-569214012997" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss_019dc2a8-10b3-7160-8575-fd21763f6c41" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_019dc2a8-10b3-7abc-a123-569214012997" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleAmortizedCostExcludingAccruedInterestAfterAllowanceForCreditLoss_019dc2a8-10b3-7160-8575-fd21763f6c41" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019dc2a8-10b3-736f-8298-4a5a8b05e24a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_019dc2a8-10b3-7abc-a123-569214012997" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019dc2a8-10b3-736f-8298-4a5a8b05e24a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019dc2a8-10b3-7498-a5d3-728100cb1f39" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_019dc2a8-10b3-7abc-a123-569214012997" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019dc2a8-10b3-7498-a5d3-728100cb1f39" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019dc2a8-10b3-7ee4-8008-eb1a6ea73e95" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesAbstract_019dc2a8-10b3-7abc-a123-569214012997" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019dc2a8-10b3-7ee4-8008-eb1a6ea73e95" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.geron.com/role/FINANCIALINSTRUMENTSCashEquivalentsandMarketableSecuritieswithUnrealizedLossesDetails" xlink:type="simple" xlink:href="gern-20260331.xsd#FINANCIALINSTRUMENTSCashEquivalentsandMarketableSecuritieswithUnrealizedLossesDetails"/>
  <link:definitionLink xlink:role="http://www.geron.com/role/FINANCIALINSTRUMENTSCashEquivalentsandMarketableSecuritieswithUnrealizedLossesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019dc2a8-10b3-7bd5-96df-2d060631677f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019dc2a8-10b3-7743-b3ad-d1a2ab7825d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019dc2a8-10b3-7bd5-96df-2d060631677f" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019dc2a8-10b3-7743-b3ad-d1a2ab7825d4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019dc2a8-10b3-7556-afd1-40efa550402d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019dc2a8-10b3-7743-b3ad-d1a2ab7825d4" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019dc2a8-10b3-7556-afd1-40efa550402d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7556-afd1-40efa550402d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019dc2a8-10b3-7556-afd1-40efa550402d" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7556-afd1-40efa550402d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7822-b8b7-6654eed56531" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019dc2a8-10b3-7556-afd1-40efa550402d" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7822-b8b7-6654eed56531" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember_019dc2cd-a688-79a8-9c1d-38520f6571d6" xlink:href="gern-20260331.xsd#gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7822-b8b7-6654eed56531" xlink:to="loc_gern_USGovernmentSponsoredEnterprisesDebtSecuritiesDueInLessThanOneYearMember_019dc2cd-a688-79a8-9c1d-38520f6571d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_MunicipalSecuritiesDueInLessThanOneYearMember_019dc2cf-0393-738e-96d0-731e5f56f96c" xlink:href="gern-20260331.xsd#gern_MunicipalSecuritiesDueInLessThanOneYearMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7822-b8b7-6654eed56531" xlink:to="loc_gern_MunicipalSecuritiesDueInLessThanOneYearMember_019dc2cf-0393-738e-96d0-731e5f56f96c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_MunicipalSecuritiesDueInOneToTwoYearsMember_019dc2b8-7e93-72f3-862f-33a9eb1c5dfd" xlink:href="gern-20260331.xsd#gern_MunicipalSecuritiesDueInOneToTwoYearsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7822-b8b7-6654eed56531" xlink:to="loc_gern_MunicipalSecuritiesDueInOneToTwoYearsMember_019dc2b8-7e93-72f3-862f-33a9eb1c5dfd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019dc2d0-fd45-71c9-bf44-603e37a6b07e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7822-b8b7-6654eed56531" xlink:to="loc_us-gaap_CommercialPaperMember_019dc2d0-fd45-71c9-bf44-603e37a6b07e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_CommercialPaperDueInLessThanOneYearMember_019dc2a8-10b3-737b-8955-b6283577d3cb" xlink:href="gern-20260331.xsd#gern_CommercialPaperDueInLessThanOneYearMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7822-b8b7-6654eed56531" xlink:to="loc_gern_CommercialPaperDueInLessThanOneYearMember_019dc2a8-10b3-737b-8955-b6283577d3cb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_CorporateNoteSecuritiesDueInLessThanOneYearMember_019dc2a8-10b3-7d05-b067-babae2a26448" xlink:href="gern-20260331.xsd#gern_CorporateNoteSecuritiesDueInLessThanOneYearMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7822-b8b7-6654eed56531" xlink:to="loc_gern_CorporateNoteSecuritiesDueInLessThanOneYearMember_019dc2a8-10b3-7d05-b067-babae2a26448" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember_019dc2a8-10b3-7032-a109-948835483eaf" xlink:href="gern-20260331.xsd#gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019dc2a8-10b3-7822-b8b7-6654eed56531" xlink:to="loc_gern_CorporateNoteSecuritiesDueInOneToTwoYearsMember_019dc2a8-10b3-7032-a109-948835483eaf" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_019dc2a8-10b3-748e-87fe-c62b522b214e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019dc2a8-10b3-7bd5-96df-2d060631677f" xlink:to="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_019dc2a8-10b3-748e-87fe-c62b522b214e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019dc2a8-10b3-7946-abdb-8d1dbd6d33d1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_019dc2a8-10b3-748e-87fe-c62b522b214e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019dc2a8-10b3-7946-abdb-8d1dbd6d33d1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_019dc2a8-10b3-7e0b-867d-ab79ce6f620a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_019dc2a8-10b3-748e-87fe-c62b522b214e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_019dc2a8-10b3-7e0b-867d-ab79ce6f620a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_019dc2a8-10b3-71cb-9495-0ad1176ddecc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtSecuritiesAvailableforSaleUnrealizedLossPositionAbstract_019dc2a8-10b3-748e-87fe-c62b522b214e" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_019dc2a8-10b3-71cb-9495-0ad1176ddecc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019dc2a8-10b3-7f16-a58e-d7424a9b960b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019dc2a8-10b3-7bd5-96df-2d060631677f" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019dc2a8-10b3-7f16-a58e-d7424a9b960b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019dc2a8-10b3-7a6f-a242-d0b4b0adb1b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019dc2a8-10b3-7f16-a58e-d7424a9b960b" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019dc2a8-10b3-7a6f-a242-d0b4b0adb1b5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019dc2a8-10b3-7781-bde5-e23c7e1ec0b3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019dc2a8-10b3-7f16-a58e-d7424a9b960b" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019dc2a8-10b3-7781-bde5-e23c7e1ec0b3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019dc2a8-10b3-735b-974a-4d0444e0edbd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLossAbstract_019dc2a8-10b3-7f16-a58e-d7424a9b960b" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019dc2a8-10b3-735b-974a-4d0444e0edbd" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.geron.com/role/FAIRVALUEMEASUREMENTSDetails" xlink:type="simple" xlink:href="gern-20260331.xsd#FAIRVALUEMEASUREMENTSDetails"/>
  <link:definitionLink xlink:role="http://www.geron.com/role/FAIRVALUEMEASUREMENTSDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c7730-4892-7f41-a7fe-6fe20f7bc636" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c7730-4892-7fa4-8ded-c0c7958c26ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c7730-4892-7f41-a7fe-6fe20f7bc636" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c7730-4892-7fa4-8ded-c0c7958c26ac" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c7730-4892-7fe8-a261-3af30a8ae13c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c7730-4892-7fa4-8ded-c0c7958c26ac" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c7730-4892-7fe8-a261-3af30a8ae13c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c7730-4892-7fe8-a261-3af30a8ae13c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c7730-4892-7fe8-a261-3af30a8ae13c" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c7730-4892-7fe8-a261-3af30a8ae13c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c7730-4892-7dd8-8288-19be500b6a0b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c7730-4892-7fe8-a261-3af30a8ae13c" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c7730-4892-7dd8-8288-19be500b6a0b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019c7730-4892-7464-b59d-c3fe01ba560b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c7730-4892-7dd8-8288-19be500b6a0b" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019c7730-4892-7464-b59d-c3fe01ba560b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019c7730-4892-7105-a20c-5f0bada97a9e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c7730-4892-7dd8-8288-19be500b6a0b" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019c7730-4892-7105-a20c-5f0bada97a9e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019c7730-4892-7f37-b74d-5a9153a07a4e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c7730-4892-7dd8-8288-19be500b6a0b" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019c7730-4892-7f37-b74d-5a9153a07a4e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAxis_019c7730-4892-78f0-90c5-cde0df64b4a2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c7730-4892-7fa4-8ded-c0c7958c26ac" xlink:to="loc_us-gaap_CashAndCashEquivalentsAxis_019c7730-4892-78f0-90c5-cde0df64b4a2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c7730-4892-78f0-90c5-cde0df64b4a2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019c7730-4892-78f0-90c5-cde0df64b4a2" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c7730-4892-78f0-90c5-cde0df64b4a2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c7730-4892-78a2-bea4-a073ae92faeb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CashAndCashEquivalentsAxis_019c7730-4892-78f0-90c5-cde0df64b4a2" xlink:to="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c7730-4892-78a2-bea4-a073ae92faeb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MoneyMarketFundsMember_019c7730-4892-7182-9b51-606006e4352e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MoneyMarketFundsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c7730-4892-78a2-bea4-a073ae92faeb" xlink:to="loc_us-gaap_MoneyMarketFundsMember_019c7730-4892-7182-9b51-606006e4352e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_019c7730-4892-7b28-92d7-4c787650cc49" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CertificatesOfDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c7730-4892-78a2-bea4-a073ae92faeb" xlink:to="loc_us-gaap_CertificatesOfDepositMember_019c7730-4892-7b28-92d7-4c787650cc49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MunicipalBondsMember_019c7730-4892-75ec-9254-fbde420623ea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MunicipalBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestrictedCashAndCashEquivalentsCashAndCashEquivalentsMember_019c7730-4892-78a2-bea4-a073ae92faeb" xlink:to="loc_us-gaap_MunicipalBondsMember_019c7730-4892-75ec-9254-fbde420623ea" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019c7730-4892-771a-92be-fd1591320b1c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c7730-4892-7fa4-8ded-c0c7958c26ac" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019c7730-4892-771a-92be-fd1591320b1c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c7730-4892-771a-92be-fd1591320b1c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c7730-4892-771a-92be-fd1591320b1c" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c7730-4892-771a-92be-fd1591320b1c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c7730-4892-792c-91cc-9155ebfdf1b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c7730-4892-771a-92be-fd1591320b1c" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c7730-4892-792c-91cc-9155ebfdf1b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryNotesSecuritiesMember_019c7730-4892-7662-9d0d-daee55c5ba75" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USTreasuryNotesSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c7730-4892-792c-91cc-9155ebfdf1b2" xlink:to="loc_us-gaap_USTreasuryNotesSecuritiesMember_019c7730-4892-7662-9d0d-daee55c5ba75" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember_019c7730-4892-7215-8a9b-e5901c0a6b71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c7730-4892-792c-91cc-9155ebfdf1b2" xlink:to="loc_us-gaap_USGovernmentSponsoredEnterprisesDebtSecuritiesMember_019c7730-4892-7215-8a9b-e5901c0a6b71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019c7730-4892-7805-bce0-0e73200997ed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c7730-4892-792c-91cc-9155ebfdf1b2" xlink:to="loc_us-gaap_CommercialPaperMember_019c7730-4892-7805-bce0-0e73200997ed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateNoteSecuritiesMember_019c7730-4892-7024-a07e-5e2ab268f6b2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateNoteSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c7730-4892-792c-91cc-9155ebfdf1b2" xlink:to="loc_us-gaap_CorporateNoteSecuritiesMember_019c7730-4892-7024-a07e-5e2ab268f6b2" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_019dc2da-0087-7f83-99df-7479f04df53c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c7730-4892-7f41-a7fe-6fe20f7bc636" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_019dc2da-0087-7f83-99df-7479f04df53c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_CashAndCashEquivalentsAndRestrictedCashFairValueDisclosure_019c7730-4892-701d-bdef-123b45e4e6a1" xlink:href="gern-20260331.xsd#gern_CashAndCashEquivalentsAndRestrictedCashFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019dc2da-0087-7f83-99df-7479f04df53c" xlink:to="loc_gern_CashAndCashEquivalentsAndRestrictedCashFairValueDisclosure_019c7730-4892-701d-bdef-123b45e4e6a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019c7730-4892-75b7-b99b-287358fda284" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019dc2da-0087-7f83-99df-7479f04df53c" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleExcludingAccruedInterest_019c7730-4892-75b7-b99b-287358fda284" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019c7730-4892-778b-9afb-8b3f75fbd8b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019dc2da-0087-7f83-99df-7479f04df53c" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019c7730-4892-778b-9afb-8b3f75fbd8b9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019dc2da-497c-7495-9e97-d83b180f8000" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c7730-4892-7f41-a7fe-6fe20f7bc636" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019dc2da-497c-7495-9e97-d83b180f8000" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedRoyaltiesCurrentAndNoncurrent_019dc2d8-5ae1-7531-802d-ffd65bcd54da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedRoyaltiesCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019dc2da-497c-7495-9e97-d83b180f8000" xlink:to="loc_us-gaap_AccruedRoyaltiesCurrentAndNoncurrent_019dc2d8-5ae1-7531-802d-ffd65bcd54da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_019dc2d8-5ae1-70b3-8557-406f3b9ceee9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019dc2da-497c-7495-9e97-d83b180f8000" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_019dc2d8-5ae1-70b3-8557-406f3b9ceee9" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.geron.com/role/DEBTAdditionalInformationDetails" xlink:type="simple" xlink:href="gern-20260331.xsd#DEBTAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.geron.com/role/DEBTAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019c7730-4892-7291-9c20-882f15620e5e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:to="loc_us-gaap_DebtInstrumentTable_019c7730-4892-7291-9c20-882f15620e5e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CounterpartyNameAxis_019c7730-4892-71cc-bb64-f741753c0019" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_CounterpartyNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c7730-4892-7291-9c20-882f15620e5e" xlink:to="loc_srt_CounterpartyNameAxis_019c7730-4892-71cc-bb64-f741753c0019" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c7730-4892-71cc-bb64-f741753c0019_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CounterpartyNameAxis_019c7730-4892-71cc-bb64-f741753c0019" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c7730-4892-71cc-bb64-f741753c0019_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c7730-4892-7a63-aaa6-74da4c9425d2" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RepurchaseAgreementCounterpartyNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CounterpartyNameAxis_019c7730-4892-71cc-bb64-f741753c0019" xlink:to="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c7730-4892-7a63-aaa6-74da4c9425d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_BiopharmaCreditPlcMember_019c7730-4892-7589-aff3-ce2248a2b0d1" xlink:href="gern-20260331.xsd#gern_BiopharmaCreditPlcMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RepurchaseAgreementCounterpartyNameDomain_019c7730-4892-7a63-aaa6-74da4c9425d2" xlink:to="loc_gern_BiopharmaCreditPlcMember_019c7730-4892-7589-aff3-ce2248a2b0d1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_019c7730-4892-7523-b5b2-8658eac16863" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c7730-4892-7291-9c20-882f15620e5e" xlink:to="loc_us-gaap_TypeOfArrangementAxis_019c7730-4892-7523-b5b2-8658eac16863" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c7730-4892-7523-b5b2-8658eac16863_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_019c7730-4892-7523-b5b2-8658eac16863" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c7730-4892-7523-b5b2-8658eac16863_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c7730-4892-7a6b-b94e-61f1a8968148" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_019c7730-4892-7523-b5b2-8658eac16863" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c7730-4892-7a6b-b94e-61f1a8968148" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_PharmakonLoanAgreementMember_019c7730-4892-71a5-9d28-96fbb2ff2f03" xlink:href="gern-20260331.xsd#gern_PharmakonLoanAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c7730-4892-7a6b-b94e-61f1a8968148" xlink:to="loc_gern_PharmakonLoanAgreementMember_019c7730-4892-71a5-9d28-96fbb2ff2f03" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_RoyaltyPharmaRevenueParticipationRightPurchaseAgreementMember_019c7730-4892-79f7-bb02-f9849eaf775b" xlink:href="gern-20260331.xsd#gern_RoyaltyPharmaRevenueParticipationRightPurchaseAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c7730-4892-7a6b-b94e-61f1a8968148" xlink:to="loc_gern_RoyaltyPharmaRevenueParticipationRightPurchaseAgreementMember_019c7730-4892-79f7-bb02-f9849eaf775b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityAxis_019c7730-4892-7b7c-be24-18b78f3e59de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c7730-4892-7291-9c20-882f15620e5e" xlink:to="loc_us-gaap_CreditFacilityAxis_019c7730-4892-7b7c-be24-18b78f3e59de" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019c7730-4892-7b7c-be24-18b78f3e59de_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CreditFacilityAxis_019c7730-4892-7b7c-be24-18b78f3e59de" xlink:to="loc_us-gaap_CreditFacilityDomain_019c7730-4892-7b7c-be24-18b78f3e59de_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CreditFacilityDomain_019c7730-4892-7e9c-8ad6-a6ec3b9ca72c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CreditFacilityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CreditFacilityAxis_019c7730-4892-7b7c-be24-18b78f3e59de" xlink:to="loc_us-gaap_CreditFacilityDomain_019c7730-4892-7e9c-8ad6-a6ec3b9ca72c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_TrancheAMember_019c7730-4892-7583-98e9-137e1c308302" xlink:href="gern-20260331.xsd#gern_TrancheAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019c7730-4892-7e9c-8ad6-a6ec3b9ca72c" xlink:to="loc_gern_TrancheAMember_019c7730-4892-7583-98e9-137e1c308302" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_TrancheBMember_019c7730-4892-7517-8e85-ab4da3d9e7ec" xlink:href="gern-20260331.xsd#gern_TrancheBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019c7730-4892-7e9c-8ad6-a6ec3b9ca72c" xlink:to="loc_gern_TrancheBMember_019c7730-4892-7517-8e85-ab4da3d9e7ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_TrancheCMember_019c7730-4892-7137-b911-c955f7c892f1" xlink:href="gern-20260331.xsd#gern_TrancheCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019c7730-4892-7e9c-8ad6-a6ec3b9ca72c" xlink:to="loc_gern_TrancheCMember_019c7730-4892-7137-b911-c955f7c892f1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_TermLoansMember_019c7730-4892-7d67-aa07-32ade672d5c5" xlink:href="gern-20260331.xsd#gern_TermLoansMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CreditFacilityDomain_019c7730-4892-7e9c-8ad6-a6ec3b9ca72c" xlink:to="loc_gern_TermLoansMember_019c7730-4892-7d67-aa07-32ade672d5c5" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019c7730-4892-7168-a472-b8b06ece3fd3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c7730-4892-7291-9c20-882f15620e5e" xlink:to="loc_us-gaap_DebtInstrumentAxis_019c7730-4892-7168-a472-b8b06ece3fd3" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019c7730-4892-7168-a472-b8b06ece3fd3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019c7730-4892-7168-a472-b8b06ece3fd3" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019c7730-4892-7168-a472-b8b06ece3fd3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019c7730-4892-77e0-97a2-8d380bbb2d48" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019c7730-4892-7168-a472-b8b06ece3fd3" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019c7730-4892-77e0-97a2-8d380bbb2d48" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_PrepaidPriorToThirdAnniversaryOfFundingDateMember_019c7730-4892-752e-83b1-3144fa3bb27e" xlink:href="gern-20260331.xsd#gern_PrepaidPriorToThirdAnniversaryOfFundingDateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c7730-4892-77e0-97a2-8d380bbb2d48" xlink:to="loc_gern_PrepaidPriorToThirdAnniversaryOfFundingDateMember_019c7730-4892-752e-83b1-3144fa3bb27e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_PrepaidOnOrAfterThirdAnniversaryButPriorToFourthAnniversaryOfFundingDateMember_019c7730-4892-793f-81f5-30fdc5fa8560" xlink:href="gern-20260331.xsd#gern_PrepaidOnOrAfterThirdAnniversaryButPriorToFourthAnniversaryOfFundingDateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c7730-4892-77e0-97a2-8d380bbb2d48" xlink:to="loc_gern_PrepaidOnOrAfterThirdAnniversaryButPriorToFourthAnniversaryOfFundingDateMember_019c7730-4892-793f-81f5-30fdc5fa8560" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_PrepaidOnOrAfterFourthAnniversaryOfFundingDateMember_019c7730-4892-7b1a-80ce-4767ac291436" xlink:href="gern-20260331.xsd#gern_PrepaidOnOrAfterFourthAnniversaryOfFundingDateMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c7730-4892-77e0-97a2-8d380bbb2d48" xlink:to="loc_gern_PrepaidOnOrAfterFourthAnniversaryOfFundingDateMember_019c7730-4892-7b1a-80ce-4767ac291436" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019c7730-4892-770d-a74f-c03cf12fa8b6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c7730-4892-7291-9c20-882f15620e5e" xlink:to="loc_srt_RangeAxis_019c7730-4892-770d-a74f-c03cf12fa8b6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c7730-4892-770d-a74f-c03cf12fa8b6_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019c7730-4892-770d-a74f-c03cf12fa8b6" xlink:to="loc_srt_RangeMember_019c7730-4892-770d-a74f-c03cf12fa8b6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c7730-4892-7595-8716-63047cefbd12" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019c7730-4892-770d-a74f-c03cf12fa8b6" xlink:to="loc_srt_RangeMember_019c7730-4892-7595-8716-63047cefbd12" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019c7730-4892-7f45-a8a0-25717d66cc4a" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c7730-4892-7595-8716-63047cefbd12" xlink:to="loc_srt_MinimumMember_019c7730-4892-7f45-a8a0-25717d66cc4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019c7730-4892-70aa-b1c1-526ba43e0426" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c7730-4892-7595-8716-63047cefbd12" xlink:to="loc_srt_MaximumMember_019c7730-4892-70aa-b1c1-526ba43e0426" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_DebtInstrumentScenarioAxis_019c7730-4892-7687-83fa-d5d51b5f1069" xlink:href="gern-20260331.xsd#gern_DebtInstrumentScenarioAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c7730-4892-7291-9c20-882f15620e5e" xlink:to="loc_gern_DebtInstrumentScenarioAxis_019c7730-4892-7687-83fa-d5d51b5f1069" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_DebtInstrumentScenarioDomain_019c7730-4892-7687-83fa-d5d51b5f1069_default" xlink:href="gern-20260331.xsd#gern_DebtInstrumentScenarioDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_gern_DebtInstrumentScenarioAxis_019c7730-4892-7687-83fa-d5d51b5f1069" xlink:to="loc_gern_DebtInstrumentScenarioDomain_019c7730-4892-7687-83fa-d5d51b5f1069_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_DebtInstrumentScenarioDomain_019c7730-4892-7df1-a4a4-7c0e981d84e9" xlink:href="gern-20260331.xsd#gern_DebtInstrumentScenarioDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_gern_DebtInstrumentScenarioAxis_019c7730-4892-7687-83fa-d5d51b5f1069" xlink:to="loc_gern_DebtInstrumentScenarioDomain_019c7730-4892-7df1-a4a4-7c0e981d84e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_ScenarioOneMember_019c7730-4892-7420-b097-d2835eb21ac2" xlink:href="gern-20260331.xsd#gern_ScenarioOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gern_DebtInstrumentScenarioDomain_019c7730-4892-7df1-a4a4-7c0e981d84e9" xlink:to="loc_gern_ScenarioOneMember_019c7730-4892-7420-b097-d2835eb21ac2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_ScenarioTwoMember_019c7730-4892-70b8-bb17-5e754f3bccb8" xlink:href="gern-20260331.xsd#gern_ScenarioTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gern_DebtInstrumentScenarioDomain_019c7730-4892-7df1-a4a4-7c0e981d84e9" xlink:to="loc_gern_ScenarioTwoMember_019c7730-4892-70b8-bb17-5e754f3bccb8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityExpirationPeriod_019c7730-4892-7af4-8b79-52f23f1d2047" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:to="loc_us-gaap_LineOfCreditFacilityExpirationPeriod_019c7730-4892-7af4-8b79-52f23f1d2047" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019c7730-4892-7496-9410-0110f3e59ca5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:to="loc_us-gaap_LineOfCreditFacilityMaximumBorrowingCapacity_019c7730-4892-7496-9410-0110f3e59ca5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019c7730-4892-7fb9-8381-2172a0156cef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentBasisSpreadOnVariableRate1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:to="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019c7730-4892-7fb9-8381-2172a0156cef" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentVariableInterestRateTypeExtensibleEnumeration_019c7730-4892-7090-b38f-e733546d2f37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentVariableInterestRateTypeExtensibleEnumeration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentBasisSpreadOnVariableRate1_019c7730-4892-7fb9-8381-2172a0156cef" xlink:to="loc_us-gaap_DebtInstrumentVariableInterestRateTypeExtensibleEnumeration_019c7730-4892-7090-b38f-e733546d2f37" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_DebtInstrumentFloorRate_019c7730-4892-7f9a-bc43-fdbaf72bb606" xlink:href="gern-20260331.xsd#gern_DebtInstrumentFloorRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:to="loc_gern_DebtInstrumentFloorRate_019c7730-4892-7f9a-bc43-fdbaf72bb606" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019c7730-4892-7606-aa41-03ebc3a9ca55" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019c7730-4892-7606-aa41-03ebc3a9ca55" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_PercentageOfCommitmentAmountToFundLoan_019c7730-4892-7d91-a21c-2386abffd228" xlink:href="gern-20260331.xsd#gern_PercentageOfCommitmentAmountToFundLoan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:to="loc_gern_PercentageOfCommitmentAmountToFundLoan_019c7730-4892-7d91-a21c-2386abffd228" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_PrepaymentPremiumOnPercentageOfTermLoan_019c7730-4892-7d49-a32c-08906d1eac3d" xlink:href="gern-20260331.xsd#gern_PrepaymentPremiumOnPercentageOfTermLoan"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:to="loc_gern_PrepaymentPremiumOnPercentageOfTermLoan_019c7730-4892-7d49-a32c-08906d1eac3d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_UpfrontPaymentReceived_019c7730-4892-7367-b113-2c5c6469fd3f" xlink:href="gern-20260331.xsd#gern_UpfrontPaymentReceived"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:to="loc_gern_UpfrontPaymentReceived_019c7730-4892-7367-b113-2c5c6469fd3f" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_PercentageOfAnnualNetSales_019c7730-4892-711f-843c-e8076931eb34" xlink:href="gern-20260331.xsd#gern_PercentageOfAnnualNetSales"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:to="loc_gern_PercentageOfAnnualNetSales_019c7730-4892-711f-843c-e8076931eb34" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019c7730-4892-7848-9899-8988730d2f95" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019c7730-4892-7848-9899-8988730d2f95" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_DebtInstrumentRoyaltyPaymentRatio_019c7730-4892-707c-a932-3214f54ca91c" xlink:href="gern-20260331.xsd#gern_DebtInstrumentRoyaltyPaymentRatio"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7d0f-bb1d-fb75af39aa87" xlink:to="loc_gern_DebtInstrumentRoyaltyPaymentRatio_019c7730-4892-707c-a932-3214f54ca91c" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.geron.com/role/DEBTScheduleofSaleofFutureRoyaltiesDetails" xlink:type="simple" xlink:href="gern-20260331.xsd#DEBTScheduleofSaleofFutureRoyaltiesDetails"/>
  <link:definitionLink xlink:role="http://www.geron.com/role/DEBTScheduleofSaleofFutureRoyaltiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7c1f-b326-223d322c4dd7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019c7730-4892-7e4f-9b69-f17281314d17" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7c1f-b326-223d322c4dd7" xlink:to="loc_us-gaap_DebtInstrumentTable_019c7730-4892-7e4f-9b69-f17281314d17" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfArrangementAxis_019c7730-4892-7e4a-aeef-f52bba7d94a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TypeOfArrangementAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c7730-4892-7e4f-9b69-f17281314d17" xlink:to="loc_us-gaap_TypeOfArrangementAxis_019c7730-4892-7e4a-aeef-f52bba7d94a1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c7730-4892-7e4a-aeef-f52bba7d94a1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TypeOfArrangementAxis_019c7730-4892-7e4a-aeef-f52bba7d94a1" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c7730-4892-7e4a-aeef-f52bba7d94a1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c7730-4892-7462-a763-3ab862dddc65" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ArrangementsAndNonarrangementTransactionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TypeOfArrangementAxis_019c7730-4892-7e4a-aeef-f52bba7d94a1" xlink:to="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c7730-4892-7462-a763-3ab862dddc65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_RoyaltyPharmaRevenueParticipationRightPurchaseAgreementMember_019c7730-4892-7812-b31c-83820a260432" xlink:href="gern-20260331.xsd#gern_RoyaltyPharmaRevenueParticipationRightPurchaseAgreementMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ArrangementsAndNonarrangementTransactionsMember_019c7730-4892-7462-a763-3ab862dddc65" xlink:to="loc_gern_RoyaltyPharmaRevenueParticipationRightPurchaseAgreementMember_019c7730-4892-7812-b31c-83820a260432" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_RoyaltiesRollForward_019c7730-4892-7b74-a14a-d39d970b5422" xlink:href="gern-20260331.xsd#gern_RoyaltiesRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c7730-4892-7c1f-b326-223d322c4dd7" xlink:to="loc_gern_RoyaltiesRollForward_019c7730-4892-7b74-a14a-d39d970b5422" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedRoyaltiesCurrentAndNoncurrent_019c7730-4892-7cb5-9257-4a42cd595a87" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedRoyaltiesCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gern_RoyaltiesRollForward_019c7730-4892-7b74-a14a-d39d970b5422" xlink:to="loc_us-gaap_AccruedRoyaltiesCurrentAndNoncurrent_019c7730-4892-7cb5-9257-4a42cd595a87" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RoyaltyExpense_019c7730-4892-7556-8a55-19adc73b69ca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RoyaltyExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gern_RoyaltiesRollForward_019c7730-4892-7b74-a14a-d39d970b5422" xlink:to="loc_us-gaap_RoyaltyExpense_019c7730-4892-7556-8a55-19adc73b69ca" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsForRoyalties_019c7730-4892-7e88-b383-5bc96a4d671c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsForRoyalties"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_gern_RoyaltiesRollForward_019c7730-4892-7b74-a14a-d39d970b5422" xlink:to="loc_us-gaap_PaymentsForRoyalties_019c7730-4892-7e88-b383-5bc96a4d671c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedRoyaltiesCurrentAndNoncurrent_019c7730-4892-76de-94c2-f59641e0c831" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedRoyaltiesCurrentAndNoncurrent"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.geron.com/role/NETLOSSPERCOMMONSHAREDetails" xlink:type="simple" xlink:href="gern-20260331.xsd#NETLOSSPERCOMMONSHAREDetails"/>
  <link:definitionLink xlink:role="http://www.geron.com/role/NETLOSSPERCOMMONSHAREDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicLineItems_019dc31f-cb9e-7a10-bb06-cb078ccacca7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasicLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_019dc31f-cb9e-70f3-89c9-7a5ac70a7bf2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_019dc31f-cb9e-7a10-bb06-cb078ccacca7" xlink:to="loc_us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_019dc31f-cb9e-70f3-89c9-7a5ac70a7bf2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019dc31f-cb9e-79f2-9b55-12d7b71acc82" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEarningsPerShareBasicByCommonClassTable_019dc31f-cb9e-70f3-89c9-7a5ac70a7bf2" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019dc31f-cb9e-79f2-9b55-12d7b71acc82" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019dc31f-cb9e-79f2-9b55-12d7b71acc82_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019dc31f-cb9e-79f2-9b55-12d7b71acc82" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019dc31f-cb9e-79f2-9b55-12d7b71acc82_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019dc31f-cb9e-790a-8f80-c114fc93c960" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019dc31f-cb9e-79f2-9b55-12d7b71acc82" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019dc31f-cb9e-790a-8f80-c114fc93c960" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockCompensationPlanMember_019dc31f-cb9e-7f60-b8f4-e7ba8a896d3a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019dc31f-cb9e-790a-8f80-c114fc93c960" xlink:to="loc_us-gaap_StockCompensationPlanMember_019dc31f-cb9e-7f60-b8f4-e7ba8a896d3a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019dc31f-cb9e-77cf-b634-f2c519603695" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicLineItems_019dc31f-cb9e-7a10-bb06-cb078ccacca7" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019dc31f-cb9e-77cf-b634-f2c519603695" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.geron.com/role/SEGMENTREPORTINGDetails" xlink:type="simple" xlink:href="gern-20260331.xsd#SEGMENTREPORTINGDetails"/>
  <link:definitionLink xlink:role="http://www.geron.com/role/SEGMENTREPORTINGDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingAssetReconcilingItemLineItems_019c7730-4892-73fc-b94c-5507e6a859b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingAssetReconcilingItemLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReconciliationOfAssetsFromSegmentToConsolidatedTable_019c7730-4892-7c31-bf6a-2906d0b16c2a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ReconciliationOfAssetsFromSegmentToConsolidatedTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingAssetReconcilingItemLineItems_019c7730-4892-73fc-b94c-5507e6a859b9" xlink:to="loc_us-gaap_ReconciliationOfAssetsFromSegmentToConsolidatedTable_019c7730-4892-7c31-bf6a-2906d0b16c2a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019c7730-4892-7644-b9ff-4c8c2e932896" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReconciliationOfAssetsFromSegmentToConsolidatedTable_019c7730-4892-7c31-bf6a-2906d0b16c2a" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019c7730-4892-7644-b9ff-4c8c2e932896" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019c7730-4892-7644-b9ff-4c8c2e932896_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019c7730-4892-7644-b9ff-4c8c2e932896" xlink:to="loc_us-gaap_SegmentDomain_019c7730-4892-7644-b9ff-4c8c2e932896_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019c7730-4892-7c71-b758-b753b8becce0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019c7730-4892-7644-b9ff-4c8c2e932896" xlink:to="loc_us-gaap_SegmentDomain_019c7730-4892-7c71-b758-b753b8becce0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_ReportableSegmentMember_019c7730-4892-7713-a736-0b260d1acf1b" xlink:href="gern-20260331.xsd#gern_ReportableSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019c7730-4892-7c71-b758-b753b8becce0" xlink:to="loc_gern_ReportableSegmentMember_019c7730-4892-7713-a736-0b260d1acf1b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019c7730-4892-70e9-85fe-cf531cfb5675" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReconciliationOfAssetsFromSegmentToConsolidatedTable_019c7730-4892-7c31-bf6a-2906d0b16c2a" xlink:to="loc_srt_ProductOrServiceAxis_019c7730-4892-70e9-85fe-cf531cfb5675" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c7730-4892-70e9-85fe-cf531cfb5675_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019c7730-4892-70e9-85fe-cf531cfb5675" xlink:to="loc_srt_ProductsAndServicesDomain_019c7730-4892-70e9-85fe-cf531cfb5675_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c7730-4892-717c-bac4-4eca9bbf1dec" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019c7730-4892-70e9-85fe-cf531cfb5675" xlink:to="loc_srt_ProductsAndServicesDomain_019c7730-4892-717c-bac4-4eca9bbf1dec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductMember_019c7730-4892-7fc4-b421-e388158a6229" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c7730-4892-717c-bac4-4eca9bbf1dec" xlink:to="loc_us-gaap_ProductMember_019c7730-4892-7fc4-b421-e388158a6229" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RoyaltyMember_019c7730-4892-7016-8fe7-5a88373fb5b8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RoyaltyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c7730-4892-717c-bac4-4eca9bbf1dec" xlink:to="loc_us-gaap_RoyaltyMember_019c7730-4892-7016-8fe7-5a88373fb5b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_ResearchAndClinicalExpensesMember_019c7730-4892-7686-ac1a-61da28d58089" xlink:href="gern-20260331.xsd#gern_ResearchAndClinicalExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c7730-4892-717c-bac4-4eca9bbf1dec" xlink:to="loc_gern_ResearchAndClinicalExpensesMember_019c7730-4892-7686-ac1a-61da28d58089" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_ChemistryManufacturingAndControlExpensesMember_019c7730-4892-7a52-b834-a3a1df11b0ae" xlink:href="gern-20260331.xsd#gern_ChemistryManufacturingAndControlExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c7730-4892-717c-bac4-4eca9bbf1dec" xlink:to="loc_gern_ChemistryManufacturingAndControlExpensesMember_019c7730-4892-7a52-b834-a3a1df11b0ae" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_gern_CommercialExpensesMember_019c7730-4892-7e96-b887-b29a485a5b56" xlink:href="gern-20260331.xsd#gern_CommercialExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c7730-4892-717c-bac4-4eca9bbf1dec" xlink:to="loc_gern_CommercialExpensesMember_019c7730-4892-7e96-b887-b29a485a5b56" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_019dc322-c5ec-719f-bf0c-a7921deb88b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReconciliationOfAssetsFromSegmentToConsolidatedTable_019c7730-4892-7c31-bf6a-2906d0b16c2a" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_019dc322-c5ec-719f-bf0c-a7921deb88b9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_019dc322-c5ec-719f-bf0c-a7921deb88b9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_019dc322-c5ec-719f-bf0c-a7921deb88b9" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_019dc322-c5ec-719f-bf0c-a7921deb88b9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_019dc322-c5ec-711d-92af-2a6ded63e17b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_019dc322-c5ec-719f-bf0c-a7921deb88b9" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_019dc322-c5ec-711d-92af-2a6ded63e17b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_019dc322-c5ec-76c7-b6aa-ccf6dfc13448" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019dc322-c5ec-711d-92af-2a6ded63e17b" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_019dc322-c5ec-76c7-b6aa-ccf6dfc13448" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_019dc324-48d3-782c-86a5-3651c49b5636" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingGeneralAndAdministrativeExpensesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019dc322-c5ec-711d-92af-2a6ded63e17b" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpensesMember_019dc324-48d3-782c-86a5-3651c49b5636" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenuesAbstract_019c7730-4892-7f9d-98fb-ac1f772ef985" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenuesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingAssetReconcilingItemLineItems_019c7730-4892-73fc-b94c-5507e6a859b9" xlink:to="loc_us-gaap_RevenuesAbstract_019c7730-4892-7f9d-98fb-ac1f772ef985" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c7730-4892-71b5-8150-961cf9f28521" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenuesAbstract_019c7730-4892-7f9d-98fb-ac1f772ef985" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c7730-4892-71b5-8150-961cf9f28521" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpensesAbstract_019c7730-4892-7277-ac16-7282733d715f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingAssetReconcilingItemLineItems_019c7730-4892-73fc-b94c-5507e6a859b9" xlink:to="loc_us-gaap_OperatingExpensesAbstract_019c7730-4892-7277-ac16-7282733d715f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingCostsAndExpenses_019c7730-4892-78ca-8f0b-fc7502091ba5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingCostsAndExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c7730-4892-7277-ac16-7282733d715f" xlink:to="loc_us-gaap_OperatingCostsAndExpenses_019c7730-4892-78ca-8f0b-fc7502091ba5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_019c7730-4892-7b10-a12a-35ca734f8b7b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c7730-4892-7277-ac16-7282733d715f" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_019c7730-4892-7b10-a12a-35ca734f8b7b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost_019c7730-4892-7026-a4a7-15e093873fd4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_019c7730-4892-7b10-a12a-35ca734f8b7b" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseExcludingAcquiredInProcessCost_019c7730-4892-7026-a4a7-15e093873fd4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_019dc322-c5ec-7fc3-94a4-0962501e6603" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringCharges"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ResearchAndDevelopmentExpenseAbstract_019c7730-4892-7b10-a12a-35ca734f8b7b" xlink:to="loc_us-gaap_RestructuringCharges_019dc322-c5ec-7fc3-94a4-0962501e6603" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpenseAbstract_019c7730-4892-7bf2-908e-0f7b7cce1b86" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingGeneralAndAdministrativeExpenseAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c7730-4892-7277-ac16-7282733d715f" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpenseAbstract_019c7730-4892-7bf2-908e-0f7b7cce1b86" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingGeneralAndAdministrativeExpense_019c7730-4892-7fcd-9688-4af05dcaad65" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SellingGeneralAndAdministrativeExpenseAbstract_019c7730-4892-7bf2-908e-0f7b7cce1b86" xlink:to="loc_us-gaap_SellingGeneralAndAdministrativeExpense_019c7730-4892-7fcd-9688-4af05dcaad65" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestructuringCharges_019dc326-7d8e-7c78-8d9e-3ba44e8d44de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestructuringCharges"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingOtherItemAmount_019c7730-4892-76da-8cb8-e84221e0a355" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingOtherItemAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c7730-4892-7277-ac16-7282733d715f" xlink:to="loc_us-gaap_SegmentReportingOtherItemAmount_019c7730-4892-76da-8cb8-e84221e0a355" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_019c7730-4892-7ada-a4e8-a59a6751de55" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestIncomeExpenseNonoperatingNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c7730-4892-7277-ac16-7282733d715f" xlink:to="loc_us-gaap_InterestIncomeExpenseNonoperatingNet_019c7730-4892-7ada-a4e8-a59a6751de55" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019c7730-4892-76b3-9e72-22960ada2312" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c7730-4892-7277-ac16-7282733d715f" xlink:to="loc_us-gaap_NetIncomeLoss_019c7730-4892-76b3-9e72-22960ada2312" xlink:type="arc" order="6"/>
  </link:definitionLink>
</link:linkbase>
