diff --git a/core/lib/Drupal/Core/Entity/ContentEntityDatabaseStorage.php b/core/lib/Drupal/Core/Entity/ContentEntityDatabaseStorage.php
index 6b7de03..7432e87 100644
--- a/core/lib/Drupal/Core/Entity/ContentEntityDatabaseStorage.php
+++ b/core/lib/Drupal/Core/Entity/ContentEntityDatabaseStorage.php
@@ -455,7 +455,7 @@ protected function postLoad(array &$queried_entities) {
     $queried_entities = $this->mapFromStorageRecords($queried_entities);
 
     // Attach field values.
-    if ($this->entityType->isFieldable()) {
+    if ($this->entityType->isExtendable()) {
       $this->loadFieldItems($queried_entities);
     }
 
diff --git a/core/lib/Drupal/Core/Entity/EntityType.php b/core/lib/Drupal/Core/Entity/EntityType.php
index 93f6062..ede279a 100644
--- a/core/lib/Drupal/Core/Entity/EntityType.php
+++ b/core/lib/Drupal/Core/Entity/EntityType.php
@@ -87,11 +87,11 @@ class EntityType implements EntityTypeInterface {
   protected $permission_granularity;
 
   /**
-   * Indicates whether fields can be attached to entities of this type.
+   * Indicates whether fields can be extendable.
    *
    * @var bool (optional)
    */
-  protected $fieldable;
+  protected $extendable;
 
   /**
    * Link templates using the URI template syntax.
@@ -438,8 +438,8 @@ public function getPermissionGranularity() {
   /**
    * {@inheritdoc}
    */
-  public function isFieldable() {
-    return isset($this->fieldable) ? $this->fieldable : FALSE;
+  public function isExtendable() {
+    return isset($this->extendable) ? $this->extendable : FALSE;
   }
 
   /**
diff --git a/core/lib/Drupal/Core/Entity/EntityTypeInterface.php b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
index 17d9555..218fe22 100644
--- a/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
+++ b/core/lib/Drupal/Core/Entity/EntityTypeInterface.php
@@ -378,9 +378,9 @@ public function getPermissionGranularity();
    * Indicates whether fields can be attached to entities of this type.
    *
    * @return bool
-   *   Returns TRUE if the entity type can has fields, otherwise FALSE.
+   *   Returns TRUE if the entity can extendable, otherwise FALSE.
    */
-  public function isFieldable();
+  public function isExtendable();
 
   /**
    * Returns link templates using the URI template syntax.
diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Entity/Feed.php b/core/modules/aggregator/lib/Drupal/aggregator/Entity/Feed.php
index ff93c30..ce8e62d 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Entity/Feed.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Entity/Feed.php
@@ -35,7 +35,7 @@
  *     "delete-form" = "aggregator.feed_delete",
  *   },
  *   base_table = "aggregator_feed",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   entity_keys = {
  *     "id" = "fid",
  *     "label" = "title",
diff --git a/core/modules/aggregator/lib/Drupal/aggregator/Entity/Item.php b/core/modules/aggregator/lib/Drupal/aggregator/Entity/Item.php
index f12ed40..e6a053b 100644
--- a/core/modules/aggregator/lib/Drupal/aggregator/Entity/Item.php
+++ b/core/modules/aggregator/lib/Drupal/aggregator/Entity/Item.php
@@ -24,7 +24,7 @@
  *     "view_builder" = "Drupal\aggregator\ItemViewBuilder"
  *   },
  *   base_table = "aggregator_item",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   entity_keys = {
  *     "id" = "iid",
  *     "label" = "title",
diff --git a/core/modules/block/custom_block/lib/Drupal/custom_block/Entity/CustomBlock.php b/core/modules/block/custom_block/lib/Drupal/custom_block/Entity/CustomBlock.php
index 421e3b5..913a013 100644
--- a/core/modules/block/custom_block/lib/Drupal/custom_block/Entity/CustomBlock.php
+++ b/core/modules/block/custom_block/lib/Drupal/custom_block/Entity/CustomBlock.php
@@ -41,7 +41,7 @@
  *     "edit-form" = "custom_block.edit",
  *     "admin-form" = "custom_block.type_edit"
  *   },
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   translatable = TRUE,
  *   entity_keys = {
  *     "id" = "id",
diff --git a/core/modules/block/lib/Drupal/block/Entity/Block.php b/core/modules/block/lib/Drupal/block/Entity/Block.php
index 9e17171..b9b845c 100644
--- a/core/modules/block/lib/Drupal/block/Entity/Block.php
+++ b/core/modules/block/lib/Drupal/block/Entity/Block.php
@@ -31,7 +31,7 @@
  *     }
  *   },
  *   admin_permission = "administer blocks",
- *   fieldable = FALSE,
+ *   extendable = FALSE,
  *   entity_keys = {
  *     "id" = "id",
  *     "label" = "label"
diff --git a/core/modules/comment/comment.module b/core/modules/comment/comment.module
index c3b4981..c26c9fb 100644
--- a/core/modules/comment/comment.module
+++ b/core/modules/comment/comment.module
@@ -890,7 +890,7 @@ function comment_entity_predelete(EntityInterface $entity) {
   // mismatched types. So, we need to verify that the ID is numeric (even for an
   // entity type that has an integer ID, $entity->id() might be a string
   // containing a number), and then cast it to an integer when querying.
-  if ($entity->getEntityType()->isFieldable() && is_numeric($entity->id())) {
+  if ($entity->getEntityType()->isExtendable() && is_numeric($entity->id())) {
     $cids = db_select('comment', 'c')
       ->fields('c', array('cid'))
       ->condition('entity_id', (int) $entity->id())
diff --git a/core/modules/comment/comment.views.inc b/core/modules/comment/comment.views.inc
index 6429f28..e14731e 100644
--- a/core/modules/comment/comment.views.inc
+++ b/core/modules/comment/comment.views.inc
@@ -353,7 +353,7 @@ function comment_views_data() {
 
   // Provide a relationship for each entity type except comment.
   foreach ($entities_types as $type => $entity_type) {
-    if ($type == 'comment' || !$entity_type->isFieldable() || !$entity_type->getBaseTable()) {
+    if ($type == 'comment' || !$entity_type->isExtendable() || !$entity_type->getBaseTable()) {
       continue;
     }
     if ($fields = \Drupal::service('comment.manager')->getFields($type)) {
@@ -432,7 +432,7 @@ function comment_views_data() {
 
   // Provide a relationship for each entity type except comment.
   foreach ($entities_types as $type => $entity_type) {
-    if ($type == 'comment' || !$entity_type->isFieldable() || !$entity_type->getBaseTable()) {
+    if ($type == 'comment' || !$entity_type->isExtendable() || !$entity_type->getBaseTable()) {
       continue;
     }
     // This relationship does not use the 'field id' column, if the entity has
@@ -604,7 +604,7 @@ function comment_views_data_alter(&$data) {
 
   // Provide a integration for each entity type except comment.
   foreach (\Drupal::entityManager()->getDefinitions() as $entity_type_id => $entity_type) {
-    if ($entity_type_id == 'comment' || !$entity_type->isFieldable() || !$entity_type->getBaseTable()) {
+    if ($entity_type_id == 'comment' || !$entity_type->isExtendable() || !$entity_type->getBaseTable()) {
       continue;
     }
     $fields = \Drupal::service('comment.manager')->getFields($entity_type_id);
diff --git a/core/modules/comment/lib/Drupal/comment/Entity/Comment.php b/core/modules/comment/lib/Drupal/comment/Entity/Comment.php
index 82a3dab..aa1210c 100644
--- a/core/modules/comment/lib/Drupal/comment/Entity/Comment.php
+++ b/core/modules/comment/lib/Drupal/comment/Entity/Comment.php
@@ -36,7 +36,7 @@
  *   },
  *   base_table = "comment",
  *   uri_callback = "comment_uri",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   translatable = TRUE,
  *   entity_keys = {
  *     "id" = "cid",
diff --git a/core/modules/config_translation/config_translation.api.php b/core/modules/config_translation/config_translation.api.php
index a7db185..a840a8f 100644
--- a/core/modules/config_translation/config_translation.api.php
+++ b/core/modules/config_translation/config_translation.api.php
@@ -38,7 +38,7 @@ function hook_config_translation_info(&$info) {
   // If field UI is not enabled, the base routes of the type
   // "field_ui.instance_edit_$entity_type" are not defined.
   if (\Drupal::moduleHandler()->moduleExists('field_ui')) {
-    // Add fields entity mappers to all fieldable entity types defined.
+    // Add fields entity mappers to all extendable entity types defined.
     foreach ($entity_manager->getDefinitions() as $entity_type_id => $entity_type) {
       $base_route = NULL;
       try {
@@ -48,8 +48,8 @@ function hook_config_translation_info(&$info) {
         // Ignore non-existent routes.
       }
 
-      // Make sure entity type is fieldable and has a base route.
-      if ($entity_type->isFieldable() && !empty($base_route)) {
+      // Make sure entity type is extendable and has a base route.
+      if ($entity_type->isExtendable() && !empty($base_route)) {
         $info[$entity_type_id . '_fields'] = array(
           'base_route_name' => 'field_ui.instance_edit_' . $entity_type_id,
           'entity_type' => 'field_instance_config',
diff --git a/core/modules/config_translation/config_translation.module b/core/modules/config_translation/config_translation.module
index 0db169e..a11f3b0 100644
--- a/core/modules/config_translation/config_translation.module
+++ b/core/modules/config_translation/config_translation.module
@@ -92,7 +92,7 @@ function config_translation_config_translation_info(&$info) {
   // If field UI is not enabled, the base routes of the type
   // "field_ui.instance_edit_$entity_type" are not defined.
   if (\Drupal::moduleHandler()->moduleExists('field_ui')) {
-    // Add fields entity mappers to all fieldable entity types defined.
+    // Add fields entity mappers to all extendable entity types defined.
     foreach ($entity_manager->getDefinitions() as $entity_type_id => $entity_type) {
       $base_route = NULL;
       try {
@@ -102,8 +102,8 @@ function config_translation_config_translation_info(&$info) {
         // Ignore non-existent routes.
       }
 
-      // Make sure entity type is fieldable and has a base route.
-      if ($entity_type->isFieldable() && !empty($base_route)) {
+      // Make sure entity type is extendable and has a base route.
+      if ($entity_type->isExtendable() && !empty($base_route)) {
         $info[$entity_type_id . '_fields'] = array(
           'base_route_name' => 'field_ui.instance_edit_' . $entity_type_id,
           'entity_type' => 'field_instance_config',
diff --git a/core/modules/contact/lib/Drupal/contact/Entity/Message.php b/core/modules/contact/lib/Drupal/contact/Entity/Message.php
index ff35420..d3d7b3e 100644
--- a/core/modules/contact/lib/Drupal/contact/Entity/Message.php
+++ b/core/modules/contact/lib/Drupal/contact/Entity/Message.php
@@ -29,7 +29,7 @@
  *     "bundle" = "category"
  *   },
  *   bundle_entity_type = "contact_category",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   links = {
  *     "admin-form" = "contact.category_edit"
  *   }
diff --git a/core/modules/content_translation/content_translation.admin.inc b/core/modules/content_translation/content_translation.admin.inc
index 557f06e..82daf8f 100644
--- a/core/modules/content_translation/content_translation.admin.inc
+++ b/core/modules/content_translation/content_translation.admin.inc
@@ -88,7 +88,7 @@ function _content_translation_form_language_content_settings_form_alter(array &$
 
       // Only show the checkbox to enable translation if the bundles in the
       // entity might have fields and if there are fields to translate.
-      if ($entity_type->isFieldable()) {
+      if ($entity_type->isExtendable()) {
         $fields = $entity_manager->getFieldDefinitions($entity_type_id, $bundle);
         if ($fields) {
           $form['settings'][$entity_type_id][$bundle]['translatable'] = array(
diff --git a/core/modules/entity/lib/Drupal/entity/Controller/EntityDisplayModeController.php b/core/modules/entity/lib/Drupal/entity/Controller/EntityDisplayModeController.php
index f0db942..71a0b13 100644
--- a/core/modules/entity/lib/Drupal/entity/Controller/EntityDisplayModeController.php
+++ b/core/modules/entity/lib/Drupal/entity/Controller/EntityDisplayModeController.php
@@ -23,7 +23,7 @@ class EntityDisplayModeController extends ControllerBase {
   public function viewModeTypeSelection() {
     $entity_types = array();
     foreach ($this->entityManager()->getDefinitions() as $entity_type_id => $entity_type) {
-      if ($entity_type->isFieldable() && $entity_type->hasViewBuilderClass()) {
+      if ($entity_type->isExtendable() && $entity_type->hasViewBuilderClass()) {
         $entity_types[$entity_type_id] = array(
           'title' => $entity_type->getLabel(),
           'link_path' => 'admin/structure/display-modes/view/add/' . $entity_type_id,
@@ -46,7 +46,7 @@ public function viewModeTypeSelection() {
   public function formModeTypeSelection() {
     $entity_types = array();
     foreach ($this->entityManager()->getDefinitions() as $entity_type_id => $entity_type) {
-      if ($entity_type->isFieldable() && $entity_type->hasFormClasses()) {
+      if ($entity_type->isExtendable() && $entity_type->hasFormClasses()) {
         $entity_types[$entity_type_id] = array(
           'title' => $entity_type->getLabel(),
           'link_path' => 'admin/structure/display-modes/form/add/' . $entity_type_id,
diff --git a/core/modules/entity/lib/Drupal/entity/EntityDisplayModeListBuilder.php b/core/modules/entity/lib/Drupal/entity/EntityDisplayModeListBuilder.php
index 05c241a..ec27fcd 100644
--- a/core/modules/entity/lib/Drupal/entity/EntityDisplayModeListBuilder.php
+++ b/core/modules/entity/lib/Drupal/entity/EntityDisplayModeListBuilder.php
@@ -93,7 +93,7 @@ public function render() {
       }
 
       // Filter entities
-      if ($this->entityTypes[$entity_type]->isFieldable() && !$this->isValidEntity($entity_type)) {
+      if ($this->entityTypes[$entity_type]->isExtendable() && !$this->isValidEntity($entity_type)) {
         continue;
       }
 
diff --git a/core/modules/entity/lib/Drupal/entity/Form/EntityDisplayModeAddForm.php b/core/modules/entity/lib/Drupal/entity/Form/EntityDisplayModeAddForm.php
index 49be44f..7a287b4 100644
--- a/core/modules/entity/lib/Drupal/entity/Form/EntityDisplayModeAddForm.php
+++ b/core/modules/entity/lib/Drupal/entity/Form/EntityDisplayModeAddForm.php
@@ -48,7 +48,7 @@ public function validate(array $form, array &$form_state) {
    */
   protected function prepareEntity() {
     $definition = $this->entityManager->getDefinition($this->targetEntityTypeId);
-    if (!$definition->isFieldable() || !$definition->hasViewBuilderClass()) {
+    if (!$definition->isExtendable() || !$definition->hasViewBuilderClass()) {
       throw new NotFoundHttpException();
     }
 
diff --git a/core/modules/entity/lib/Drupal/entity/Form/EntityFormModeAddForm.php b/core/modules/entity/lib/Drupal/entity/Form/EntityFormModeAddForm.php
index 4cdece5..cee7dbb 100644
--- a/core/modules/entity/lib/Drupal/entity/Form/EntityFormModeAddForm.php
+++ b/core/modules/entity/lib/Drupal/entity/Form/EntityFormModeAddForm.php
@@ -19,7 +19,7 @@ class EntityFormModeAddForm extends EntityDisplayModeAddForm {
    */
   protected function prepareEntity() {
     $definition = $this->entityManager->getDefinition($this->targetEntityTypeId);
-    if (!$definition->isFieldable() || !$definition->hasFormClasses()) {
+    if (!$definition->isExtendable() || !$definition->hasFormClasses()) {
       throw new NotFoundHttpException();
     }
 
diff --git a/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php b/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php
index e0b6e0e..8d1bfcf 100644
--- a/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php
+++ b/core/modules/entity_reference/lib/Drupal/entity_reference/Plugin/Field/FieldFormatter/EntityReferenceFormatterBase.php
@@ -55,7 +55,7 @@ public function prepareView(array $entities_items) {
       }
     }
 
-    // Iterate through the fieldable entities again to attach the loaded data.
+    // Iterate through the extendable entities again to attach the loaded data.
     foreach ($entities_items as $items) {
       $rekey = FALSE;
       foreach ($items as $item) {
diff --git a/core/modules/field/field.module b/core/modules/field/field.module
index db35e40..915cbbc 100644
--- a/core/modules/field/field.module
+++ b/core/modules/field/field.module
@@ -27,7 +27,7 @@
  *
  * The Field API allows custom data fields to be attached to Drupal entities and
  * takes care of storing, loading, editing, and rendering field data. Any entity
- * type (node, user, etc.) can use the Field API to make itself "fieldable" and
+ * type (node, user, etc.) can use the Field API to make itself "extendable" and
  * thus allow fields to be attached to it. Other modules can provide a user
  * interface for managing custom fields via a web browser as well as a wide and
  * flexible variety of data type, form element, and display format capabilities.
@@ -36,7 +36,7 @@
  * the concept of a Bundle. A Field defines a particular type of data that can
  * be attached to entities. A Field Instance is a Field attached to a single
  * Bundle. A Bundle is a set of fields that are treated as a group by the Field
- * Attach API and is related to a single fieldable entity type.
+ * Attach API and is related to a single extendable entity type.
  *
  * For example, suppose a site administrator wants Article nodes to have a
  * subtitle and photo. Using the Field API or Field UI module, the administrator
@@ -60,8 +60,8 @@
  * - @link field_attach Field Attach API @endlink: Connects entity types to the
  *   Field API. Field Attach API functions load, store, generate Form API
  *   structures, display, and perform a variety of other functions for field
- *   data connected to individual entities. Fieldable entity types like node and
- *   user use this API to make themselves fieldable.
+ *   data connected to individual entities. Extendable entity types like node and
+ *   user use this API to make themselves extendable.
  *
  * - @link field_info Field Info API @endlink: Exposes information about all
  *   fields, instances, widgets, and related information defined by or with the
@@ -82,7 +82,7 @@ function field_help($path, $arg) {
     case 'admin/help#field':
       $output = '';
       $output .= '<h3>' . t('About') . '</h3>';
-      $output .= '<p>' . t('The Field module allows custom data fields to be defined for <em>entity</em> types (entities include content items, comments, user accounts, and taxonomy terms). The Field module takes care of storing, loading, editing, and rendering field data. Most users will not interact with the Field module directly, but will instead use the <a href="@field-ui-help">Field UI module</a> user interface. Module developers can use the Field API to make new entity types "fieldable" and thus allow fields to be attached to them. For more information, see the online handbook entry for <a href="@field">Field module</a>.', array('@field-ui-help' => url('admin/help/field_ui'), '@field' => 'http://drupal.org/documentation/modules/field')) . '</p>';
+      $output .= '<p>' . t('The Field module allows custom data fields to be defined for <em>entity</em> types (entities include content items, comments, user accounts, and taxonomy terms). The Field module takes care of storing, loading, editing, and rendering field data. Most users will not interact with the Field module directly, but will instead use the <a href="@field-ui-help">Field UI module</a> user interface. Module developers can use the Field API to make new entity types "extendable" and thus allow fields to be attached to them. For more information, see the online handbook entry for <a href="@field">Field module</a>.', array('@field-ui-help' => url('admin/help/field_ui'), '@field' => 'http://drupal.org/documentation/modules/field')) . '</p>';
       $output .= '<h3>' . t('Uses') . '</h3>';
       $output .= '<dl>';
       $output .= '<dt>' . t('Enabling field types') . '</dt>';
@@ -177,7 +177,7 @@ function field_system_info_alter(&$info, Extension $file, $type) {
  * Implements hook_entity_bundle_field_info().
  */
 function field_entity_bundle_field_info(EntityTypeInterface $entity_type, $bundle, array $base_field_definitions) {
-  if ($entity_type->isFieldable()) {
+  if ($entity_type->isExtendable()) {
     // Configurable fields, which are always attached to a specific bundle, are
     // added 'by bundle'.
     return Field::fieldInfo()->getBundleInstances($entity_type->id(), $bundle);
diff --git a/core/modules/field_ui/field_ui.module b/core/modules/field_ui/field_ui.module
index 21772e1..81631ac 100644
--- a/core/modules/field_ui/field_ui.module
+++ b/core/modules/field_ui/field_ui.module
@@ -2,7 +2,7 @@
 
 /**
  * @file
- * Allows administrators to attach custom fields to fieldable types.
+ * Allows administrators to attach custom fields to extendable types.
  */
 
 use Drupal\Core\Entity\EntityInterface;
@@ -60,8 +60,8 @@ function field_ui_permission() {
   $permissions = array();
 
   foreach (\Drupal::entityManager()->getDefinitions() as $entity_type_id => $entity_type) {
-    if ($entity_type->isFieldable()) {
-      // Create a permission for each fieldable entity to manage
+    if ($entity_type->isExtendable()) {
+      // Create a permission for each extendable entity to manage
       // the fields and the display.
       $permissions['administer ' . $entity_type_id . ' fields'] = array(
         'title' => t('%entity_label: Administer fields', array('%entity_label' => $entity_type->getLabel())),
diff --git a/core/modules/field_ui/lib/Drupal/field_ui/Plugin/Derivative/FieldUiLocalTask.php b/core/modules/field_ui/lib/Drupal/field_ui/Plugin/Derivative/FieldUiLocalTask.php
index f7fd862..3fd7d48 100644
--- a/core/modules/field_ui/lib/Drupal/field_ui/Plugin/Derivative/FieldUiLocalTask.php
+++ b/core/modules/field_ui/lib/Drupal/field_ui/Plugin/Derivative/FieldUiLocalTask.php
@@ -74,7 +74,7 @@ public function getDerivativeDefinitions($base_plugin_definition) {
     $this->derivatives = array();
 
     foreach ($this->entityManager->getDefinitions() as $entity_type_id => $entity_type) {
-      if ($entity_type->isFieldable() && $entity_type->hasLinkTemplate('admin-form')) {
+      if ($entity_type->isExtendable() && $entity_type->hasLinkTemplate('admin-form')) {
         $this->derivatives["overview_$entity_type_id"] = array(
           'route_name' => "field_ui.overview_$entity_type_id",
           'weight' => 1,
@@ -177,7 +177,7 @@ public function getDerivativeDefinitions($base_plugin_definition) {
    */
   public function alterLocalTasks(&$local_tasks) {
     foreach ($this->entityManager->getDefinitions() as $entity_type => $entity_info) {
-      if ($entity_info->isFieldable() && $entity_info->hasLinkTemplate('admin-form')) {
+      if ($entity_info->isExtendable() && $entity_info->hasLinkTemplate('admin-form')) {
         $admin_form = $entity_info->getLinkTemplate('admin-form');
         $local_tasks["field_ui.fields:overview_$entity_type"]['base_route'] = $admin_form;
         $local_tasks["field_ui.fields:form_display_overview_$entity_type"]['base_route'] = $admin_form;
diff --git a/core/modules/field_ui/lib/Drupal/field_ui/Routing/RouteSubscriber.php b/core/modules/field_ui/lib/Drupal/field_ui/Routing/RouteSubscriber.php
index 14b1b6b..73a6d86 100644
--- a/core/modules/field_ui/lib/Drupal/field_ui/Routing/RouteSubscriber.php
+++ b/core/modules/field_ui/lib/Drupal/field_ui/Routing/RouteSubscriber.php
@@ -41,7 +41,7 @@ public function __construct(EntityManagerInterface $manager) {
   protected function alterRoutes(RouteCollection $collection, $provider) {
     foreach ($this->manager->getDefinitions() as $entity_type_id => $entity_type) {
       $defaults = array();
-      if ($entity_type->isFieldable() && $entity_type->hasLinkTemplate('admin-form')) {
+      if ($entity_type->isExtendable() && $entity_type->hasLinkTemplate('admin-form')) {
         // Try to get the route from the current collection.
         if (!$entity_route = $collection->get($entity_type->getLinkTemplate('admin-form'))) {
           continue;
diff --git a/core/modules/node/lib/Drupal/node/Entity/Node.php b/core/modules/node/lib/Drupal/node/Entity/Node.php
index 0e1d20f..8f9326c 100644
--- a/core/modules/node/lib/Drupal/node/Entity/Node.php
+++ b/core/modules/node/lib/Drupal/node/Entity/Node.php
@@ -39,7 +39,7 @@
  *   revision_table = "node_revision",
  *   revision_data_table = "node_field_revision",
  *   uri_callback = "node_uri",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   translatable = TRUE,
  *   entity_keys = {
  *     "id" = "nid",
diff --git a/core/modules/rest/lib/Drupal/rest/LinkManager/RelationLinkManager.php b/core/modules/rest/lib/Drupal/rest/LinkManager/RelationLinkManager.php
index 8cf4afa..e000908 100644
--- a/core/modules/rest/lib/Drupal/rest/LinkManager/RelationLinkManager.php
+++ b/core/modules/rest/lib/Drupal/rest/LinkManager/RelationLinkManager.php
@@ -87,7 +87,7 @@ protected function writeCache() {
     $data = array();
 
     foreach ($this->entityManager->getDefinitions() as $entity_type) {
-      if ($entity_type->isFieldable()) {
+      if (in_array('isExtendable', get_class_methods($entity_type))) {
         foreach ($this->entityManager->getBundleInfo($entity_type->id()) as $bundle => $bundle_info) {
           foreach ($this->entityManager->getFieldDefinitions($entity_type->id(), $bundle) as $field_definition) {
             $relation_uri = $this->getRelationUri($entity_type->id(), $bundle, $field_definition->getName());
diff --git a/core/modules/system/lib/Drupal/system/Tests/Entity/EntityCacheTagsTestBase.php b/core/modules/system/lib/Drupal/system/Tests/Entity/EntityCacheTagsTestBase.php
index e1f66b5..9b63a321 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Entity/EntityCacheTagsTestBase.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Entity/EntityCacheTagsTestBase.php
@@ -60,10 +60,10 @@ public function setUp() {
     // Create an entity.
     $this->entity = $this->createEntity();
 
-    // If this is a fieldable entity, then add a configurable field. We will use
+    // If this is a extendable entity, then add a configurable field. We will use
     // this configurable field in later tests to ensure that modifications to
     // field (instance) configuration invalidate render cache entries.
-    if ($this->entity->getEntityType()->isFieldable()) {
+    if ($this->entity->getEntityType()->isExtendable()) {
       // Add field, so we can modify the Field and FieldInstance entities to
       // verify that changes to those indeed clear cache tags.
       $field_name = drupal_strtolower($this->randomName());
@@ -337,7 +337,7 @@ public function testReferencedEntity() {
     }
 
 
-    if ($this->entity->getEntityType()->isFieldable()) {
+    if ($this->entity->getEntityType()->isExtendable()) {
       // Verify that after modifying a configurable field on the entity, there
       // is a cache miss.
       $this->pass("Test modification of referenced entity's configurable field.", 'Debug');
diff --git a/core/modules/system/lib/Drupal/system/Tests/Entity/EntityWithUriCacheTagsTestBase.php b/core/modules/system/lib/Drupal/system/Tests/Entity/EntityWithUriCacheTagsTestBase.php
index 3d308a5..fd77b57 100644
--- a/core/modules/system/lib/Drupal/system/Tests/Entity/EntityWithUriCacheTagsTestBase.php
+++ b/core/modules/system/lib/Drupal/system/Tests/Entity/EntityWithUriCacheTagsTestBase.php
@@ -96,7 +96,7 @@ public function testEntityUri() {
     }
 
 
-    if ($this->entity->getEntityType()->isFieldable()) {
+    if ($this->entity->getEntityType()->isExtendable()) {
       // Verify that after modifying a configurable field on the entity, there
       // is a cache miss.
       $this->pass("Test modification of entity's configurable field.", 'Debug');
diff --git a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTest.php b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTest.php
index fb1087f..b0b1d20 100644
--- a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTest.php
+++ b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTest.php
@@ -31,7 +31,7 @@
  *     "translation" = "Drupal\content_translation\ContentTranslationController"
  *   },
  *   base_table = "entity_test",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   field_cache = FALSE,
  *   entity_keys = {
  *     "id" = "id",
diff --git a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestBaseFieldDisplay.php b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestBaseFieldDisplay.php
index c76cff7..b5d74a4 100644
--- a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestBaseFieldDisplay.php
+++ b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestBaseFieldDisplay.php
@@ -24,7 +24,7 @@
  *     "translation" = "Drupal\content_translation\ContentTranslationController"
  *   },
  *   base_table = "entity_test",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   entity_keys = {
  *     "id" = "id",
  *     "uuid" = "uuid",
diff --git a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestCache.php b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestCache.php
index 6e509b5..7970c9a 100644
--- a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestCache.php
+++ b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestCache.php
@@ -23,7 +23,7 @@
  *     "translation" = "Drupal\content_translation\ContentTranslationController"
  *   },
  *   base_table = "entity_test",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   field_cache = TRUE,
  *   entity_keys = {
  *     "id" = "id",
diff --git a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestLabelCallback.php b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestLabelCallback.php
index 481711f..4c1484a 100644
--- a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestLabelCallback.php
+++ b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestLabelCallback.php
@@ -17,7 +17,7 @@
  *   base_table = "entity_test",
  *   revision_table = "entity_test_revision",
  *   label_callback = "entity_test_label_callback",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   entity_keys = {
  *     "id" = "id",
  *     "bundle" = "type"
diff --git a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestMul.php b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestMul.php
index 49f7d5f..c640e48 100644
--- a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestMul.php
+++ b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestMul.php
@@ -27,7 +27,7 @@
  *   },
  *   base_table = "entity_test_mul",
  *   data_table = "entity_test_mul_property_data",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   translatable = TRUE,
  *   entity_keys = {
  *     "id" = "id",
diff --git a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestMulRev.php b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestMulRev.php
index 1e6dc44..e394341 100644
--- a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestMulRev.php
+++ b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestMulRev.php
@@ -28,7 +28,7 @@
  *   data_table = "entity_test_mulrev_property_data",
  *   revision_table = "entity_test_mulrev_revision",
  *   revision_data_table = "entity_test_mulrev_property_revision",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   translatable = TRUE,
  *   entity_keys = {
  *     "id" = "id",
diff --git a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestRev.php b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestRev.php
index 8be477a..3c5b501 100644
--- a/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestRev.php
+++ b/core/modules/system/tests/modules/entity_test/lib/Drupal/entity_test/Entity/EntityTestRev.php
@@ -26,7 +26,7 @@
  *   },
  *   base_table = "entity_test_rev",
  *   revision_table = "entity_test_rev_revision",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   entity_keys = {
  *     "id" = "id",
  *     "uuid" = "uuid",
diff --git a/core/modules/taxonomy/lib/Drupal/taxonomy/Entity/Term.php b/core/modules/taxonomy/lib/Drupal/taxonomy/Entity/Term.php
index bbbed9c..d6629dc 100644
--- a/core/modules/taxonomy/lib/Drupal/taxonomy/Entity/Term.php
+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/Entity/Term.php
@@ -34,7 +34,7 @@
  *   },
  *   base_table = "taxonomy_term_data",
  *   uri_callback = "taxonomy_term_uri",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   translatable = TRUE,
  *   entity_keys = {
  *     "id" = "tid",
diff --git a/core/modules/taxonomy/lib/Drupal/taxonomy/Plugin/Field/FieldFormatter/TaxonomyFormatterBase.php b/core/modules/taxonomy/lib/Drupal/taxonomy/Plugin/Field/FieldFormatter/TaxonomyFormatterBase.php
index 29beaf5..5077133 100644
--- a/core/modules/taxonomy/lib/Drupal/taxonomy/Plugin/Field/FieldFormatter/TaxonomyFormatterBase.php
+++ b/core/modules/taxonomy/lib/Drupal/taxonomy/Plugin/Field/FieldFormatter/TaxonomyFormatterBase.php
@@ -23,7 +23,7 @@
   public function prepareView(array $entities_items) {
     $tids = array();
 
-    // Collect every possible term attached to any of the fieldable entities.
+    // Collect every possible term attached to any of the extendable entities.
     foreach ($entities_items as $items) {
       foreach ($items as $item) {
         // Force the array key to prevent duplicates.
@@ -35,7 +35,7 @@ public function prepareView(array $entities_items) {
     if ($tids) {
       $terms = entity_load_multiple('taxonomy_term', $tids);
 
-      // Iterate through the fieldable entities again to attach the loaded term
+      // Iterate through the extendable entities again to attach the loaded term
       // data.
       foreach ($entities_items as $items) {
         $rekey = FALSE;
diff --git a/core/modules/taxonomy/taxonomy.module b/core/modules/taxonomy/taxonomy.module
index d570ced..5f9225a 100644
--- a/core/modules/taxonomy/taxonomy.module
+++ b/core/modules/taxonomy/taxonomy.module
@@ -746,7 +746,7 @@ function taxonomy_autocomplete_validate($element, &$form_state) {
  * Functions to maintain taxonomy indexing.
  *
  * Taxonomy uses default field storage to store canonical relationships
- * between terms and fieldable entities. However its most common use case
+ * between terms and extendable entities. However its most common use case
  * requires listing all content associated with a term or group of terms
  * sorted by creation date. To avoid slow queries due to joining across
  * multiple node and field tables with various conditions and order by criteria,
diff --git a/core/modules/user/lib/Drupal/user/Entity/User.php b/core/modules/user/lib/Drupal/user/Entity/User.php
index fc33baf..3a69753 100644
--- a/core/modules/user/lib/Drupal/user/Entity/User.php
+++ b/core/modules/user/lib/Drupal/user/Entity/User.php
@@ -37,7 +37,7 @@
  *   base_table = "users",
  *   uri_callback = "user_uri",
  *   label_callback = "user_format_name",
- *   fieldable = TRUE,
+ *   extendable = TRUE,
  *   translatable = TRUE,
  *   entity_keys = {
  *     "id" = "uid",