diff --git a/core/includes/theme.inc b/core/includes/theme.inc
index 0384456..4e37a2a 100644
--- a/core/includes/theme.inc
+++ b/core/includes/theme.inc
@@ -830,38 +830,20 @@ function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  * engine, it overrides the default implementation of the 'page' theme hook by
  * containing a 'page.html.twig' file within its folder structure).
  *
- * @todo Add/edit/remove these subsections with hook_theme_prepare().
- * @subsection sub_preprocess_templates Preprocessing for Template Files
- * If the implementation is a template file, several functions are called before
- * the template file is invoked to modify the $variables array. These make up
- * the "preprocessing" phase, and are executed (if they exist), in the following
- * order (note that in the following list, HOOK indicates the theme hook name,
- * MODULE indicates a module name, THEME indicates a theme name, and ENGINE
- * indicates a theme engine name):
- * - template_preprocess(&$variables, $hook): Creates a default set of variables
- *   for all theme hooks with template implementations.
- * - template_preprocess_HOOK(&$variables): Should be implemented by the module
- *   that registers the theme hook, to set up default variables.
- * - MODULE_preprocess(&$variables, $hook): hook_preprocess() is invoked on all
- *   implementing modules.
- * - MODULE_preprocess_HOOK(&$variables): hook_preprocess_HOOK() is invoked on
- *   all implementing modules, so that modules that didn't define the theme hook
- *   can alter the variables.
- * - ENGINE_engine_preprocess(&$variables, $hook): Allows the theme engine to
- *   set necessary variables for all theme hooks with template implementations.
- * - ENGINE_engine_preprocess_HOOK(&$variables): Allows the theme engine to set
- *   necessary variables for the particular theme hook.
- * - THEME_preprocess(&$variables, $hook): Allows the theme to set necessary
- *   variables for all theme hooks with template implementations.
- * - THEME_preprocess_HOOK(&$variables): Allows the theme to set necessary
- *   variables specific to the particular theme hook.
- *
- * @subsection sub_preprocess_theme_funcs Preprocessing for Theme Functions
- * If the implementation is a function, only the theme-hook-specific preprocess
- * functions (the ones ending in _HOOK) are called from the list above. This is
- * because theme hooks with function implementations need to be fast, and
- * calling the non-theme-hook-specific preprocess functions for them would incur
- * a noticeable performance penalty.
+ * @subsection sub_prepare_variables Preparing Variables for Theme Hooks
+ * Several functions are invoked before theme functions or template files are
+ * provided their available $variables. These functions make up the "prepare"
+ * phase, and are executed (if they exist), in the following
+ * order (HOOK indicates the profile, module, theme engine or theme name,
+ * THEME_HOOK indicates the name of the specific theme hook called):
+ * - HOOK_theme_prepare(&$variables, $context): Prepare a set of variables
+ *   for all theme hook implementations.
+ * - HOOK_theme_prepare_THEME_HOOK(&$variables, $context): Prepare a set of
+ *   variables for a specific theme hook implementation.
+ * - HOOK_theme_prepare_alter(&$variables, $context): Alter a set of prepared
+ *   variables for all theme hook implementations.
+ * - HOOK_theme_prepare_THEME_HOOK_alter(&$variables, $context): Alter a set of
+ *   prepared variables for a specific theme hook implementation.
  *
  * @subsection sub_alternate_suggestions Suggesting Alternate Hooks
  * Alternate hooks can be suggested by implementing the hook-specific
@@ -877,11 +859,11 @@ function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  *
  * @param $hook
  *   The name of the theme hook to call. If the name contains a
- *   double-underscore ('__') and there isn't an implementation for the full
- *   name, the part before the '__' is checked. This allows a fallback to a
+ *   double-underscore ('__') and there is not an implementation for the full
+ *   name, the name before the '__' is checked. This allows a fallback to a
  *   more generic implementation. For example, if theme('links__node', ...) is
  *   called, but there is no implementation of that theme hook, then the
- *   'links' implementation is used. This process is iterative, so if
+ *   'links' implementation is used instead. This process is iterative, so if
  *   theme('links__contextual__node', ...) is called, theme() checks for the
  *   following implementations, and uses the first one that exists:
  *   - links__contextual__node
@@ -895,10 +877,14 @@ function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  *   convention is not desired or is insufficient.
  * @param $variables
  *   An associative array of variables to merge with defaults from the theme
- *   registry, pass to preprocess functions for modification, and finally, pass
+ *   registry, pass to prepare functions for modification, and finally, pass
  *   to the function or template implementing the theme hook. Alternatively,
  *   this can be a renderable array, in which case, its properties are mapped to
  *   variables expected by the theme hook implementations.
+ * @param array $context
+ *   An associative array of data used as context in hook_theme_prepare(). This
+ *   array is used internally and is not available as a "variable" in  theme
+ *   functions or templates.
  *
  * @return string|false
  *   An HTML string representing the themed output or FALSE if the passed $hook
@@ -911,7 +897,7 @@ function drupal_find_base_themes($themes, $key, $used_keys = array()) {
  *
  * @throws Exception
  */
-function theme($hook, $variables = array()) {
+function theme($hook, $variables = array(), $context = array()) {
   static $default_attributes;
   // If called before all modules are loaded, we do not necessarily have a full
   // theme registry to work with, and therefore cannot process the theme
diff --git a/core/modules/system/tests/modules/theme_test/theme_test.module b/core/modules/system/tests/modules/theme_test/theme_test.module
index 8e55fc5..bab08b3 100644
--- a/core/modules/system/tests/modules/theme_test/theme_test.module
+++ b/core/modules/system/tests/modules/theme_test/theme_test.module
@@ -194,7 +194,7 @@ function theme_theme_test_prepare_alter_variable($variables) {
  */
 function theme_test_theme_prepare($context) {
   $variables = array();
-  if ($context['theme_id'] == 'theme_test_prepare_variable' || $context['theme_id'] == 'theme_test_prepare_alter_variable') {
+  if ($context['theme_hook'] == 'theme_test_prepare_variable' || $context['theme_hook'] == 'theme_test_prepare_alter_variable') {
     $variables['element'] = array(
       'variable1' => array(
         '#markup' => '<p>Inserted variable1 via hook_theme_prepare().</p>',
@@ -234,7 +234,7 @@ function theme_test_theme_prepare_theme_test_prepare_alter_variable() {
  * Implements hook_theme_prepare_alter().
  */
 function theme_test_theme_prepare_alter(&$variables, $context) {
-  if ($context['theme_id'] == 'theme_test_prepare_alter_variable') {
+  if ($context['theme_hook'] == 'theme_test_prepare_alter_variable') {
     $variables['element']['variable1']['#markup'] = '<p>Inserted variable1 via hook_theme_prepare() was altered by hook_theme_prepare_alter().</p>';
   }
 }
diff --git a/core/modules/system/tests/themes/test_theme/test_theme.theme b/core/modules/system/tests/themes/test_theme/test_theme.theme
index ac559ec..2fec7d3 100644
--- a/core/modules/system/tests/themes/test_theme/test_theme.theme
+++ b/core/modules/system/tests/themes/test_theme/test_theme.theme
@@ -75,7 +75,7 @@ function test_theme_theme_test_function_suggestions__module_override($variables)
  */
 function test_theme_theme_prepare($context) {
   $variables = array();
-  if ($context['theme_id'] == 'theme_test_prepare_variable' || $context['theme_id'] == 'theme_test_prepare_alter_variable') {
+  if ($context['theme_hook'] == 'theme_test_prepare_variable' || $context['theme_hook'] == 'theme_test_prepare_alter_variable') {
     $variables['element'] = array(
       'themeVariable1' => array(
         '#markup' => '<p>Inserted themeVariable1 via theme hook_theme_prepare().</p>',
@@ -86,7 +86,7 @@ function test_theme_theme_prepare($context) {
 }

 /**
- * Implements hook_theme_prepare_BASE_THEME_ID().
+ * Implements hook_theme_prepare_THEME_HOOK().
  */
 function test_theme_theme_prepare_theme_test_prepare_variable() {
   $variables = array();
@@ -99,7 +99,7 @@ function test_theme_theme_prepare_theme_test_prepare_variable() {
 }

 /**
- * Implements hook_theme_prepare_BASE_THEME_ID().
+ * Implements hook_theme_prepare_THEME_HOOK().
  */
 function test_theme_theme_prepare_theme_test_prepare_alter_variable() {
   $variables = array();
@@ -115,7 +115,7 @@ function test_theme_theme_prepare_theme_test_prepare_alter_variable() {
  * Implements hook_theme_prepare_alter().
  */
 function test_theme_theme_prepare_alter(&$variables, $context) {
-  if ($context['theme_id'] == 'theme_test_prepare_alter_variable') {
+  if ($context['theme_hook'] == 'theme_test_prepare_alter_variable') {
     $variables['element']['themeVariable1']['#markup'] = '<p>Inserted themeVariable1 via theme hook_theme_prepare() was altered by theme hook_theme_prepare_alter().</p>';
   }
 }