diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index c63f679..fbbf81a 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -124,8 +124,8 @@ function drupal_theme_initialize() {
  * @param \Drupal\Core\Extension\Extension $theme
  *   The theme extension object.
  * @param \Drupal\Core\Extension\Extension[] $base_theme
- *    An optional array of objects that represent the 'base theme' if the
- *    theme is meant to be derivative of another theme. It requires
+ *    An optional array of objects that represents the list of 'base' themes if
+ *    the theme is meant to be derivative of another theme. It requires
  *    the same information as the $theme object. It should be in
  *    'oldest first' order, meaning the top level of the chain will
  *    be first.
diff --git a/core/lib/Drupal/Core/Extension/InfoParserInterface.php b/core/lib/Drupal/Core/Extension/InfoParserInterface.php
index 2c6e2d1..9acb330 100644
--- a/core/lib/Drupal/Core/Extension/InfoParserInterface.php
+++ b/core/lib/Drupal/Core/Extension/InfoParserInterface.php
@@ -35,7 +35,7 @@
    * Information stored in a theme .info.yml file:
    * - screenshot: Path to screenshot relative to the theme's .info.yml file.
    * - engine: Theme engine; typically twig.
-   * - base theme: Name of a base theme, if applicable.
+   * - base: Name of a base theme, if applicable.
    * - regions: Listed regions.
    * - features: Features available.
    * - stylesheets: Theme stylesheets.
diff --git a/core/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
index b43d3e4..fd349a6 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -222,8 +222,8 @@ public function listInfo() {
         if (isset($theme->info['engine'])) {
           $theme->engine = $theme->info['engine'];
         }
-        if (isset($theme->info['base theme'])) {
-          $theme->base_theme = $theme->info['base theme'];
+        if (isset($theme->info['base'])) {
+          $theme->base_theme = $theme->info['base'];
         }
         // Status is normally retrieved from the database. Add zero values when
         // read from the installation directory to prevent notices.
@@ -293,7 +293,7 @@ public function rebuildThemeData() {
       $type = 'theme';
       $this->moduleHandler->alter('system_info', $theme->info, $theme, $type);
 
-      if (!empty($theme->info['base theme'])) {
+      if (!empty($theme->info['base'])) {
         $sub_themes[] = $key;
       }
 
@@ -399,11 +399,11 @@ public function getBaseThemes(array $themes, $theme) {
    *   An array of base themes.
    */
   protected function doGetBaseThemes(array $themes, $theme, $used_themes = array()) {
-    if (!isset($themes[$theme]->info['base theme'])) {
+    if (!isset($themes[$theme]->info['base'])) {
       return array();
     }
 
-    $base_key = $themes[$theme]->info['base theme'];
+    $base_key = $themes[$theme]->info['base'];
     // Does the base theme exist?
     if (!isset($themes[$base_key])) {
       return array($base_key => NULL);
@@ -412,7 +412,7 @@ protected function doGetBaseThemes(array $themes, $theme, $used_themes = array()
     $current_base_theme = array($base_key => $themes[$base_key]->info['name']);
 
     // Is the base theme itself a child of another theme?
-    if (isset($themes[$base_key]->info['base theme'])) {
+    if (isset($themes[$base_key]->info['base'])) {
       // Do we already know the base themes of this theme?
       if (isset($themes[$base_key]->base_themes)) {
         return $themes[$base_key]->base_themes + $current_base_theme;
diff --git a/core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php b/core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php
index 2ba52cd..7feff72 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandlerInterface.php
@@ -55,7 +55,7 @@ public function disable(array $theme_list);
    *     in the .info.yml file.
    *   - prefix: The base theme engine prefix.
    *   - engine: The machine name of the theme engine.
-   *   - base_theme: If this is a sub-theme, the machine name of the base theme
+   *   - base: If this is a sub-theme, the machine name of the base theme
    *     defined in the .info.yml file. Otherwise, the element is not set.
    *   - base_themes: If this is a sub-theme, an associative array of the
    *     base-theme ancestors of this theme, starting with this theme's base
diff --git a/core/modules/breakpoint/tests/themes/breakpoint_test_theme/breakpoint_test_theme.info.yml b/core/modules/breakpoint/tests/themes/breakpoint_test_theme/breakpoint_test_theme.info.yml
index 0fa9096..8cbe1b6 100644
--- a/core/modules/breakpoint/tests/themes/breakpoint_test_theme/breakpoint_test_theme.info.yml
+++ b/core/modules/breakpoint/tests/themes/breakpoint_test_theme/breakpoint_test_theme.info.yml
@@ -3,5 +3,5 @@ type: theme
 description: 'Test theme for breakpoint.'
 version: VERSION
 core: 8.x
-'base theme': bartik
+base: bartik
 hidden: true
diff --git a/core/modules/ckeditor/ckeditor.module b/core/modules/ckeditor/ckeditor.module
index 7000dbf..b776d30 100644
--- a/core/modules/ckeditor/ckeditor.module
+++ b/core/modules/ckeditor/ckeditor.module
@@ -79,8 +79,8 @@ function _ckeditor_theme_css($theme = NULL) {
         $css[$key] = $theme_path . '/' . $path;
       }
     }
-    if (isset($info['base theme'])) {
-      $css = array_merge(_ckeditor_theme_css($info['base theme'], $css));
+    if (isset($info['base'])) {
+      $css = array_merge(_ckeditor_theme_css($info['base'], $css));
     }
   }
   return $css;
diff --git a/core/modules/edit/edit.module b/core/modules/edit/edit.module
index 6c8e90f..dd1c249 100644
--- a/core/modules/edit/edit.module
+++ b/core/modules/edit/edit.module
@@ -80,8 +80,8 @@ function edit_library_alter(array &$library, $name, $theme = NULL) {
     if ($theme && $theme_path = drupal_get_path('theme', $theme)) {
       $info = system_get_info('theme', $theme);
       // Recurse to process base theme(s) first.
-      if (isset($info['base theme'])) {
-        edit_library_alter($library, $name, $info['base theme']);
+      if (isset($info['base'])) {
+        edit_library_alter($library, $name, $info['base']);
       }
       if (isset($info['edit_stylesheets']) && is_array($info['edit_stylesheets'])) {
         foreach ($info['edit_stylesheets'] as $path) {
diff --git a/core/modules/system/lib/Drupal/system/Controller/SystemController.php b/core/modules/system/lib/Drupal/system/Controller/SystemController.php
index 421791f..7f51fae 100644
--- a/core/modules/system/lib/Drupal/system/Controller/SystemController.php
+++ b/core/modules/system/lib/Drupal/system/Controller/SystemController.php
@@ -230,7 +230,7 @@ public function themesPage() {
         $theme->incompatible_core = !isset($theme->info['core']) || ($theme->info['core'] != \DRUPAL::CORE_COMPATIBILITY) || !isset($theme->info['regions']['content']);
         $theme->incompatible_php = version_compare(phpversion(), $theme->info['php']) < 0;
         // Confirmed that the base theme is available.
-        $theme->incompatible_base = isset($theme->info['base theme']) && !isset($themes[$theme->info['base theme']]);
+        $theme->incompatible_base = isset($theme->info['base']) && !isset($themes[$theme->info['base']]);
         // Confirm that the theme engine is available.
         $theme->incompatible_engine = isset($theme->info['engine']) && !isset($theme->owner);
       }
diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index 8cc1599..5f8307d 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -472,7 +472,7 @@ function template_preprocess_system_themes_page(&$variables) {
         $current_theme['incompatible'] = t('This theme requires PHP version @php_required and is incompatible with PHP version !php_version.', array('@php_required' => $theme->info['php'], '!php_version' => phpversion()));
       }
       elseif (!empty($theme->incompatible_base)) {
-        $current_theme['incompatible'] = t('This theme requires the base theme @base_theme to operate correctly.', array('@base_theme' => $theme->info['base theme']));
+        $current_theme['incompatible'] = t('This theme requires the base theme @base_theme to operate correctly.', array('@base_theme' => $theme->info['base']));
       }
       elseif (!empty($theme->incompatible_engine)) {
         $current_theme['incompatible'] = t('This theme requires the theme engine @theme_engine to operate correctly.', array('@theme_engine' => $theme->info['engine']));
diff --git a/core/modules/system/tests/themes/test_invalid_basetheme/test_invalid_basetheme.info.yml b/core/modules/system/tests/themes/test_invalid_basetheme/test_invalid_basetheme.info.yml
index a8d2bfe..b08f3fd 100644
--- a/core/modules/system/tests/themes/test_invalid_basetheme/test_invalid_basetheme.info.yml
+++ b/core/modules/system/tests/themes/test_invalid_basetheme/test_invalid_basetheme.info.yml
@@ -3,5 +3,5 @@ type: theme
 description: 'Test theme which has a non-existent base theme.'
 version: VERSION
 core: 8.x
-'base theme': not_real_test_basetheme
+base: not_real_test_basetheme
 hidden: true
diff --git a/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml b/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml
index 6123684..984398d 100644
--- a/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml
+++ b/core/modules/system/tests/themes/test_subtheme/test_subtheme.info.yml
@@ -3,7 +3,7 @@ type: theme
 description: 'Test theme which uses test_basetheme as the base theme.'
 version: VERSION
 core: 8.x
-'base theme': test_basetheme
+base: test_basetheme
 hidden: true
 stylesheets:
   all:
diff --git a/core/modules/update/tests/themes/update_test_subtheme/update_test_subtheme.info.yml b/core/modules/update/tests/themes/update_test_subtheme/update_test_subtheme.info.yml
index d844542..fe349fa 100644
--- a/core/modules/update/tests/themes/update_test_subtheme/update_test_subtheme.info.yml
+++ b/core/modules/update/tests/themes/update_test_subtheme/update_test_subtheme.info.yml
@@ -3,5 +3,5 @@ type: theme
 description: 'Test theme which uses update_test_basetheme as the base theme.'
 version: VERSION
 core: 8.x
-'base theme': update_test_basetheme
+base: update_test_basetheme
 hidden: true
diff --git a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
index 3864923..070c4d1 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
@@ -208,7 +208,7 @@ public function testEnableAndListInfo() {
         'example/theme',
       ),
       'engine' => 'twig',
-      'base theme' => 'stark',
+      'base' => 'stark',
     );
 
     $list_info = $this->themeHandler->listInfo();
@@ -341,7 +341,7 @@ public function testRebuildThemeDataWithThemeParents() {
     $this->assertEquals('test_subtheme', $info_subtheme->getName());
 
     // Test the parent/child-theme properties.
-    $info_subtheme->info['base theme'] = 'test_basetheme';
+    $info_subtheme->info['base'] = 'test_basetheme';
     $info_basetheme->sub_themes = array('test_subtheme');
 
     $this->assertEquals(DRUPAL_ROOT . '/core/themes/engines/twig/twig.engine', $info_basetheme->owner);
@@ -392,7 +392,7 @@ public function providerTestGetBaseThemes() {
       'name' => 'test_1',
       'info' => array(
         'name' => 'test_1',
-        'base theme' => 'test_2',
+        'base' => 'test_2',
       ),
     );
     $data[] = array($themes, 'test_1', array('test_2' => NULL));
@@ -403,7 +403,7 @@ public function providerTestGetBaseThemes() {
       'name' => 'test_1',
       'info' => array(
         'name' => 'test_1',
-        'base theme' => 'test_2',
+        'base' => 'test_2',
       ),
     );
     $themes['test_2'] = (object) array(
@@ -420,14 +420,14 @@ public function providerTestGetBaseThemes() {
       'name' => 'test_1',
       'info' => array(
         'name' => 'test_1',
-        'base theme' => 'test_2',
+        'base' => 'test_2',
       ),
     );
     $themes['test_2'] = (object) array(
       'name' => 'test_2',
       'info' => array(
         'name' => 'test_2',
-        'base theme' => 'test_3',
+        'base' => 'test_3',
       ),
     );
     $themes['test_3'] = (object) array(