diff --git a/stix2/test/v21/test_custom.py b/stix2/test/v21/test_custom.py
index a2e41b2..c9c9f5d 100644
--- a/stix2/test/v21/test_custom.py
+++ b/stix2/test/v21/test_custom.py
@@ -1449,7 +1449,7 @@ def test_registered_new_extension_sdo_allow_custom_false():
         'some_property_name1': 'value1',
         'some_property_name2': 'value2',
         # 'extensions': {
-        #     'extension-definition--d83fce45-ef58-4c6c-a3f4-1fbc32e9999': STIXExtensiond83fce45ef584c6ca3f41fbc32e98c6e()
+        #     'extension-definition--d83fce45-ef58-4c6c-a3f4-1fbc32e9999': ExtensionDefinitiond83fce45ef584c6ca3f41fbc32e98c6e()
         # }
     }
     sdo_object = stix2.parse(my_favorite_sdo)
diff --git a/stix2/v21/common.py b/stix2/v21/common.py
index 4410975..0a6929e 100644
--- a/stix2/v21/common.py
+++ b/stix2/v21/common.py
@@ -259,7 +259,7 @@ def CustomMarking(type='x-custom-marking', properties=None, extension_name=None)
 
             extension = extension_name.split('--')[1]
             extension = extension.replace('-', '')
-            NameExtension.__name__ = 'STIXExtension' + extension
+            NameExtension.__name__ = 'ExtensionDefinition' + extension
             cls.with_extension = extension_name
         return _custom_marking_builder(cls, type, properties, '2.1', _STIXBase21)
     return wrapper
diff --git a/stix2/v21/observables.py b/stix2/v21/observables.py
index 84ac136..b30455d 100644
--- a/stix2/v21/observables.py
+++ b/stix2/v21/observables.py
@@ -950,7 +950,7 @@ def CustomObservable(type='x-custom-observable', properties=None, id_contrib_pro
 
             extension = extension_name.split('--')[1]
             extension = extension.replace('-', '')
-            NameExtension.__name__ = 'STIXExtension' + extension
+            NameExtension.__name__ = 'ExtensionDefinition' + extension
             cls.with_extension = extension_name
         return _custom_observable_builder(cls, type, _properties, '2.1', _Observable, id_contrib_props)
     return wrapper
diff --git a/stix2/v21/sdo.py b/stix2/v21/sdo.py
index af22fc5..63e2d13 100644
--- a/stix2/v21/sdo.py
+++ b/stix2/v21/sdo.py
@@ -866,7 +866,7 @@ def CustomObject(type='x-custom-type', properties=None, extension_name=None):
 
             extension = extension_name.split('--')[1]
             extension = extension.replace('-', '')
-            NameExtension.__name__ = 'STIXExtension' + extension
+            NameExtension.__name__ = 'ExtensionDefinition' + extension
             cls.with_extension = extension_name
         return _custom_object_builder(cls, type, _properties, '2.1', _DomainObject)