diff --git a/core/lib/Drupal/Core/Extension/ThemeHandler.php b/core/lib/Drupal/Core/Extension/ThemeHandler.php
index f42f1bd..4e8cda8 100644
--- a/core/lib/Drupal/Core/Extension/ThemeHandler.php
+++ b/core/lib/Drupal/Core/Extension/ThemeHandler.php
@@ -400,8 +400,8 @@ public function addTheme(Extension $theme) {
     if (isset($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['parent'])) {
+      $theme->base_theme = $theme->info['parent'];
     }
     $this->list[$theme->getName()] = $theme;
   }
@@ -484,10 +484,10 @@ public function rebuildThemeData() {
       $type = 'theme';
       $this->moduleHandler->alter('system_info', $theme->info, $theme, $type);
 
-      if (!empty($theme->info['base theme'])) {
+      if (!empty($theme->info['parent'])) {
         $sub_themes[] = $key;
-        // Add the base theme as a proper dependency.
-        $themes[$key]->info['dependencies'][] = $themes[$key]->info['base theme'];
+        // Add the parent theme as a proper dependency.
+        $themes[$key]->info['dependencies'][] = $themes[$key]->info['parent'];
       }
 
       // Defaults to 'twig' (see $defaults above).
@@ -602,11 +602,11 @@ 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['parent'])) {
       return array();
     }
 
-    $base_key = $themes[$theme]->info['base theme'];
+    $base_key = $themes[$theme]->info['parent'];
     // Does the base theme exist?
     if (!isset($themes[$base_key])) {
       return array($base_key => NULL);
@@ -614,8 +614,8 @@ 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'])) {
+    // Is the parent theme itself a child of another theme?
+    if (isset($themes[$base_key]->info['parent'])) {
       // Do we already know the base themes of this theme?
       if (isset($themes[$base_key]->base_themes)) {
         return $themes[$base_key]->base_themes + $current_base_theme;
diff --git a/core/modules/breakpoint/tests/themes/breakpoint_theme_test/breakpoint_theme_test.info.yml b/core/modules/breakpoint/tests/themes/breakpoint_theme_test/breakpoint_theme_test.info.yml
index 779f997..76803a1 100644
--- a/core/modules/breakpoint/tests/themes/breakpoint_theme_test/breakpoint_theme_test.info.yml
+++ b/core/modules/breakpoint/tests/themes/breakpoint_theme_test/breakpoint_theme_test.info.yml
@@ -3,4 +3,4 @@ type: theme
 description: 'Test theme for breakpoint.'
 version: VERSION
 core: 8.x
-'base theme': bartik
+parent: bartik
diff --git a/core/modules/ckeditor/ckeditor.module b/core/modules/ckeditor/ckeditor.module
index 5cb8ef3..cefefcf 100644
--- a/core/modules/ckeditor/ckeditor.module
+++ b/core/modules/ckeditor/ckeditor.module
@@ -91,8 +91,8 @@ function _ckeditor_theme_css($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['parent'])) {
+      $css = array_merge(_ckeditor_theme_css($info['parent'], $css));
     }
   }
   return $css;
diff --git a/core/modules/quickedit/quickedit.module b/core/modules/quickedit/quickedit.module
index 6b5a9cf..96d214a 100644
--- a/core/modules/quickedit/quickedit.module
+++ b/core/modules/quickedit/quickedit.module
@@ -94,8 +94,8 @@ function quickedit_library_alter(array &$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'])) {
-        quickedit_library_alter($library, $name, $info['base theme']);
+      if (isset($info['parent'])) {
+        quickedit_library_alter($library, $name, $info['parent']);
       }
       if (isset($info['quickedit_stylesheets']) && is_array($info['quickedit_stylesheets'])) {
         foreach ($info['quickedit_stylesheets'] as $path) {
diff --git a/core/modules/system/src/Controller/SystemController.php b/core/modules/system/src/Controller/SystemController.php
index 4f1b2f1..b7a14f4 100644
--- a/core/modules/system/src/Controller/SystemController.php
+++ b/core/modules/system/src/Controller/SystemController.php
@@ -231,7 +231,7 @@ 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['parent']) && !isset($themes[$theme->info['parent']]);
         // Confirm that the theme engine is available.
         $theme->incompatible_engine = isset($theme->info['engine']) && !isset($theme->owner);
       }
diff --git a/core/modules/system/system.admin.inc b/core/modules/system/system.admin.inc
index 57edc26..cc84b7d 100644
--- a/core/modules/system/system.admin.inc
+++ b/core/modules/system/system.admin.inc
@@ -422,7 +422,7 @@ function template_preprocess_system_themes_page(&$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' => $theme->info['parent']));
       }
       elseif (!empty($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 f29a4d4..6e6377c 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
@@ -3,4 +3,4 @@ type: theme
 description: 'Test theme which has a non-existent base theme.'
 version: VERSION
 core: 8.x
-'base theme': not_real_test_basetheme
+parent: not_real_test_basetheme
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 6adfee4..fa872ed 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
+parent: test_basetheme
 stylesheets:
   all:
     - css/sub-add.css
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 2580fd6..9a8d69b 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
@@ -3,4 +3,4 @@ type: theme
 description: 'Test theme which uses update_test_basetheme as the base theme.'
 version: VERSION
 core: 8.x
-'base theme': update_test_basetheme
+parent: update_test_basetheme
diff --git a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
index 3d89abe..31c8bfb 100644
--- a/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
+++ b/core/tests/Drupal/Tests/Core/Extension/ThemeHandlerTest.php
@@ -258,7 +258,7 @@ 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['parent'] = 'test_basetheme';
     $info_basetheme->sub_themes = array('test_subtheme');
 
     $this->assertEquals(DRUPAL_ROOT . '/core/themes/engines/twig/twig.engine', $info_basetheme->owner);
@@ -293,7 +293,7 @@ public function testGetBaseThemes(array $themes, $theme, array $expected) {
   public function providerTestGetBaseThemes() {
     $data = array();
 
-    // Tests a theme without any base theme.
+    // Tests a theme without any parent theme.
     $themes = array();
     $themes['test_1'] = (object) array(
       'name' => 'test_1',
@@ -303,24 +303,24 @@ public function providerTestGetBaseThemes() {
     );
     $data[] = array($themes, 'test_1', array());
 
-    // Tests a theme with a non existing base theme.
+    // Tests a theme with a non existing parent theme.
     $themes = array();
     $themes['test_1'] = (object) array(
       'name' => 'test_1',
       'info' => array(
         'name' => 'test_1',
-        'base theme' => 'test_2',
+        'parent' => 'test_2',
       ),
     );
     $data[] = array($themes, 'test_1', array('test_2' => NULL));
 
-    // Tests a theme with a single existing base theme.
+    // Tests a theme with a single existing parent theme.
     $themes = array();
     $themes['test_1'] = (object) array(
       'name' => 'test_1',
       'info' => array(
         'name' => 'test_1',
-        'base theme' => 'test_2',
+        'parent' => 'test_2',
       ),
     );
     $themes['test_2'] = (object) array(
@@ -331,20 +331,20 @@ public function providerTestGetBaseThemes() {
     );
     $data[] = array($themes, 'test_1', array('test_2' => 'test_2'));
 
-    // Tests a theme with multiple base themes.
+    // Tests a theme with multiple parent themes.
     $themes = array();
     $themes['test_1'] = (object) array(
       'name' => 'test_1',
       'info' => array(
         'name' => 'test_1',
-        'base theme' => 'test_2',
+        'parent' => 'test_2',
       ),
     );
     $themes['test_2'] = (object) array(
       'name' => 'test_2',
       'info' => array(
         'name' => 'test_2',
-        'base theme' => 'test_3',
+        'parent' => 'test_3',
       ),
     );
     $themes['test_3'] = (object) array(