6 files declare their use of FieldDefinition

entity.api.php in core/lib/Drupal/Core/Entity/entity.api.php
Hooks and documentation related to entities.
EntityFieldManager.php in core/lib/Drupal/Core/Entity/EntityFieldManager.php
EntitySchemaTestHooks.php in core/modules/system/tests/modules/entity_schema_test/src/Hook/EntitySchemaTestHooks.php
EntityTestComputedBundleField.php in core/modules/system/tests/modules/entity_test/src/Entity/EntityTestComputedBundleField.php
entity_schema_test.module in core/modules/system/tests/modules/entity_schema_test/entity_schema_test.module
FieldDefinitionTest.php in core/tests/Drupal/Tests/Core/Entity/FieldDefinitionTest.php

Buggy or inaccurate documentation? Please file an issue. Need support? Need help programming? Connect with the Drupal community.