diff --git a/core/modules/responsive_image/config/schema/responsive_image.schema.yml b/core/modules/responsive_image/config/schema/responsive_image.schema.yml
index 75b1d83..27acfa6 100644
--- a/core/modules/responsive_image/config/schema/responsive_image.schema.yml
+++ b/core/modules/responsive_image/config/schema/responsive_image.schema.yml
@@ -10,11 +10,11 @@ responsive_image.mappings.*:
     label:
       type: label
       label: 'Label'
-    mapping_definitions:
+    mappingDefinitions:
       type: sequence
       label: 'Mapping definitions'
       sequence:
-        - type: mapping
+        - type: mapping_definition
           label: 'Mapping definition'
           mapping_definition:
             # Image mapping type. Either 'sizes' (using the 'sizes' attribute)
diff --git a/core/modules/responsive_image/src/Entity/ResponsiveImageMapping.php b/core/modules/responsive_image/src/Entity/ResponsiveImageMapping.php
index 3103e4c..9a60f23 100644
--- a/core/modules/responsive_image/src/Entity/ResponsiveImageMapping.php
+++ b/core/modules/responsive_image/src/Entity/ResponsiveImageMapping.php
@@ -57,21 +57,28 @@ class ResponsiveImageMapping extends ConfigEntityBase implements ResponsiveImage
   protected $label;
 
   /**
-   * The responsive image mappings.
+   * The responsive image mapping definitions.
    *
-   * Each responsive mapping array contains the following keys:
-   * - breakpoint_id
-   * - multiplier
-   * - image_style
+   * Each mapping definition array contains the following keys:
+   *   - image_mapping_type: Either 'image_style' or 'sizes'.
+   *   - image_mapping:
+   *     - If image_mapping_type is 'image_style', the image style ID.
+   *     - If image_mapping_type is 'sizes', an array with following keys:
+   *       - sizes: If image_mapping_type is 'sizes', the value for the
+   *         'sizes' attribute.
+   *       - sizes_image_styles: The image styles to use for the 'srcset'
+   *         attribute.
+   *   - breakpoint_id: The breakpoint ID for this mapping definition.
+   *   - multiplier: The multiplier for this mapping definition.
    *
    * @var array
    */
-  protected $mappings = array();
+  protected $mappingDefinitions = array();
 
   /**
    * @var array
    */
-  protected $keyedMappings;
+  protected $keyedMappingDefinitions;
 
   /**
    * The responsive image breakpoint group.
@@ -96,7 +103,7 @@ public function addMappingDefinition($breakpoint_id, $multiplier, array $mapping
       'image_mapping' => '',
     );
     // If there is an existing mapping, overwrite it.
-    foreach ($this->mappings as &$mapping) {
+    foreach ($this->mappingDefinitions as &$mapping) {
       if ($mapping['breakpoint_id'] === $breakpoint_id && $mapping['multiplier'] === $multiplier) {
         $mapping = array(
           'breakpoint_id' => $breakpoint_id,
@@ -105,11 +112,11 @@ public function addMappingDefinition($breakpoint_id, $multiplier, array $mapping
         return $this;
       }
     }
-    $this->mappings[] = array(
+    $this->mappingDefinitions[] = array(
       'breakpoint_id' => $breakpoint_id,
       'multiplier' => $multiplier,
     ) + $mapping_definition + $defaults;
-    $this->keyedMappings = NULL;
+    $this->keyedMappingDefinitions = NULL;
     return $this;
   }
 
@@ -125,33 +132,34 @@ public function hasMappingDefinitions() {
    * {@inheritdoc}
    */
   public function getKeyedMappingDefinitions() {
-    if (!$this->keyedMappings) {
-      $this->keyedMappings = array();
-      foreach($this->mappings as $mapping) {
+    if (!$this->keyedMappingDefinitions) {
+      $this->keyedMappingDefinitions = array();
+      foreach($this->mappingDefinitions as $mapping) {
         if (!$this->isEmptyMappingDefinition($mapping)) {
           // Only return the selected image styles.
           if ($mapping['image_mapping_type'] == 'sizes') {
             $mapping['image_mapping']['sizes_image_styles'] = array_filter($mapping['image_mapping']['sizes_image_styles']);
           }
-          $this->keyedMappings[$mapping['breakpoint_id']][$mapping['multiplier']] = $mapping;
+          $this->keyedMappingDefinitions[$mapping['breakpoint_id']][$mapping['multiplier']] = $mapping;
         }
       }
     }
-    return $this->keyedMappings;
+    return $this->keyedMappingDefinitions;
   }
 
   /**
    * {@inheritdoc}
    */
   public function getMappingDefinitions() {
-    return $this->get('mappings');
+    return $this->get('mappingDefinitions');
   }
 
   /**
    * {@inheritdoc}
    */
   public function setBreakpointGroup($breakpoint_group) {
-    // If the breakpoint group is changed then the mappings are invalid.
+    // If the breakpoint group is changed then the mapping definitions are
+    // invalid.
     if ($breakpoint_group !== $this->breakpointGroup) {
       $this->removeMappingDefinitions();
     }
@@ -170,8 +178,8 @@ public function getBreakpointGroup() {
    * {@inheritdoc}
    */
   public function removeMappingDefinitions() {
-    $this->mappings = array();
-    $this->keyedMappings = NULL;
+    $this->mappingDefinitions = array();
+    $this->keyedMappingDefinitions = NULL;
     return $this;
   }