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 f574d87..430dc47 100644
--- a/core/modules/responsive_image/config/schema/responsive_image.schema.yml
+++ b/core/modules/responsive_image/config/schema/responsive_image.schema.yml
@@ -10,12 +10,12 @@ responsive_image.mappings.*:
     label:
       type: label
       label: 'Label'
-    mappings:
+    mappingDefinitions:
       type: sequence
-      label: 'Mappings'
+      label: 'Mapping definitions'
       sequence:
         - type: mapping
-          label: 'Mapping'
+          label: 'Mapping definition'
           mapping:
             # Image mapping type. Either 'sizes' (using the 'sizes' attribute)
             # or 'image_style' (using a single image style to map to this
diff --git a/core/modules/responsive_image/responsive_image.module b/core/modules/responsive_image/responsive_image.module
index 56d9797..ee10739 100644
--- a/core/modules/responsive_image/responsive_image.module
+++ b/core/modules/responsive_image/responsive_image.module
@@ -181,7 +181,7 @@ function template_preprocess_responsive_image(&$variables) {
 
   $image = \Drupal::service('image.factory')->get($variables['uri']);
   $variables['sources'] = responsive_image_build_source_attributes($image, $variables);
-  // Output the fallback image. Use srcset in the fallback image to avoid
+  // Prepare the fallback image. Use srcset in the fallback image to avoid
   // unnecessary preloading of images in older browsers. See
   // http://scottjehl.github.io/picturefill/#using-picture and
   // http://scottjehl.github.io/picturefill/#gotchas for more information.
@@ -199,6 +199,7 @@ function template_preprocess_responsive_image(&$variables) {
     }
   }
 }
+
 /**
  * Helper function for template_preprocess_responsive_image().
  *
@@ -307,7 +308,7 @@ function responsive_image_build_source_attributes(ImageInterface $image, $variab
   $responsive_image_mapping = ResponsiveImageMapping::load($variables['mapping_id']);
   // All breakpoints and multipliers.
   $breakpoints = \Drupal::service('breakpoint.manager')->getBreakpointsByGroup($responsive_image_mapping->getBreakpointGroup());
-  foreach ($responsive_image_mapping->getKeyedMappings() as $breakpoint_id => $multipliers) {
+  foreach ($responsive_image_mapping->getKeyedMappingDefinitions() as $breakpoint_id => $multipliers) {
     if (isset($breakpoints[$breakpoint_id])) {
       $breakpoint = $breakpoints[$breakpoint_id];
       $sizes = array();
@@ -330,7 +331,12 @@ function responsive_image_build_source_attributes(ImageInterface $image, $variab
               // it. Because of this, the image styles for all multipliers of
               // this breakpoint should be merged into one srcset and the sizes
               // attribute should be merged as well.
-              $srcset[floatval($dimensions['width'])] = file_create_url(_responsive_image_image_style_url($image_style_name, $image->getSource())) . ' ' . $dimensions['width'] . 'w';
+              if (!is_null($dimensions['width'])) {
+                $srcset[floatval($dimensions['width'])] = file_create_url(_responsive_image_image_style_url($image_style_name, $image->getSource())) . ' ' . $dimensions['width'] . 'w';
+              }
+              else {
+                \Drupal::logger('responsive_image')->warning('Could not determine image width for @file using image style with ID: @image_style_name', array('@file' => $image->getSource(), '@image_style_name' => $image_style_name));
+              }
               $sizes = array_merge(explode(',', $mapping_definition['image_mapping']['sizes']), $sizes);
             }
             break;
diff --git a/core/modules/responsive_image/src/Entity/ResponsiveImageMapping.php b/core/modules/responsive_image/src/Entity/ResponsiveImageMapping.php
index c600f51..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.
@@ -90,13 +97,13 @@ public function __construct(array $values, $entity_type_id = 'responsive_image_m
   /**
    * {@inheritdoc}
    */
-  public function addMapping($breakpoint_id, $multiplier, array $mapping_definition) {
+  public function addMappingDefinition($breakpoint_id, $multiplier, array $mapping_definition) {
     $defaults = array(
       'image_mapping_type' => 'image_style',
       'image_mapping' => '',
     );
     // If there is an existing mapping, overwrite it.
-    foreach ($this->mappings as &$mapping) {
+    foreach ($this->mappingDefinitions as &$mapping) {
       if ($mapping['breakpoint_id'] === $breakpoint_id && $mapping['multiplier'] === $multiplier) {
         $mapping = array(
           'breakpoint_id' => $breakpoint_id,
@@ -105,55 +112,56 @@ public function addMapping($breakpoint_id, $multiplier, array $mapping_definitio
         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;
   }
 
   /**
    * {@inheritdoc}
    */
-  public function hasMappings() {
-    $mappings = $this->getKeyedMappings();
+  public function hasMappingDefinitions() {
+    $mappings = $this->getKeyedMappingDefinitions();
     return !empty($mappings);
   }
 
   /**
    * {@inheritdoc}
    */
-  public function getKeyedMappings() {
-    if (!$this->keyedMappings) {
-      $this->keyedMappings = array();
-      foreach($this->mappings as $mapping) {
+  public function getKeyedMappingDefinitions() {
+    if (!$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 getMappings() {
-    return $this->get('mappings');
+  public function getMappingDefinitions() {
+    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->removeMappings();
+      $this->removeMappingDefinitions();
     }
     $this->set('breakpointGroup', $breakpoint_group);
     return $this;
@@ -169,9 +177,9 @@ public function getBreakpointGroup() {
   /**
    * {@inheritdoc}
    */
-  public function removeMappings() {
-    $this->mappings = array();
-    $this->keyedMappings = NULL;
+  public function removeMappingDefinitions() {
+    $this->mappingDefinitions = array();
+    $this->keyedMappingDefinitions = NULL;
     return $this;
   }
 
@@ -215,7 +223,7 @@ public static function isEmptyMappingDefinition(array $mapping_definition) {
    * {@inheritdoc}
    */
   public function getMappingDefinition($breakpoint_id, $multiplier) {
-    $map = $this->getKeyedMappings();
+    $map = $this->getKeyedMappingDefinitions();
     if (isset($map[$breakpoint_id][$multiplier])) {
       return $map[$breakpoint_id][$multiplier];
     }
diff --git a/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php b/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php
index 5ed1e3b7..32c9eeb 100644
--- a/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php
+++ b/core/modules/responsive_image/src/Plugin/Field/FieldFormatter/ResponsiveImageFormatter.php
@@ -198,7 +198,7 @@ public function viewElements(FieldItemListInterface $items) {
     $cache_tags = [];
     if ($responsive_image_mapping) {
       $cache_tags = Cache::mergeTags($cache_tags, $responsive_image_mapping->getCacheTags());
-      foreach ($responsive_image_mapping->getMappings() as $mapping) {
+      foreach ($responsive_image_mapping->getMappingDefinitions() as $mapping) {
         // Only image styles of non-empty mappings should be loaded.
         if (!$responsive_image_mapping::isEmptyMappingDefinition($mapping)) {
 
diff --git a/core/modules/responsive_image/src/ResponsiveImageMappingForm.php b/core/modules/responsive_image/src/ResponsiveImageMappingForm.php
index 6090495..2973413 100644
--- a/core/modules/responsive_image/src/ResponsiveImageMappingForm.php
+++ b/core/modules/responsive_image/src/ResponsiveImageMappingForm.php
@@ -148,11 +148,11 @@ public function save(array $form, FormStateInterface $form_state) {
     /** @var \Drupal\responsive_image\ResponsiveImageMappingInterface $responsive_image_mapping */
     $responsive_image_mapping = $this->entity;
     // Remove all the existing mappings and replace with submitted values.
-    $responsive_image_mapping->removeMappings();
+    $responsive_image_mapping->removeMappingDefinitions();
     if ($form_state->hasValue('keyed_mappings')) {
       foreach ($form_state->getValue('keyed_mappings') as $breakpoint_id => $multipliers) {
         foreach ($multipliers as $multiplier => $mapping) {
-          $responsive_image_mapping->addMapping($breakpoint_id, $multiplier, $mapping);
+          $responsive_image_mapping->addMappingDefinition($breakpoint_id, $multiplier, $mapping);
         }
       }
     }
@@ -163,7 +163,7 @@ public function save(array $form, FormStateInterface $form_state) {
 
     // Redirect to edit form after creating a new mapping or after selecting
     // another breakpoint group.
-    if (!$responsive_image_mapping->hasMappings()) {
+    if (!$responsive_image_mapping->hasMappingDefinitions()) {
       $form_state->setRedirect(
         'entity.responsive_image_mapping.edit_form',
         array('responsive_image_mapping' => $responsive_image_mapping->id())
diff --git a/core/modules/responsive_image/src/ResponsiveImageMappingInterface.php b/core/modules/responsive_image/src/ResponsiveImageMappingInterface.php
index 7457aee..b35a727 100644
--- a/core/modules/responsive_image/src/ResponsiveImageMappingInterface.php
+++ b/core/modules/responsive_image/src/ResponsiveImageMappingInterface.php
@@ -20,7 +20,7 @@
    * return bool
    *   Whether the entity has any responsive image mappings.
    */
-  public function hasMappings();
+  public function hasMappingDefinitions();
 
   /**
    * Returns the mappings of breakpoint ID and multiplier to image style.
@@ -39,7 +39,7 @@ public function hasMappings();
    *     - breakpoint_id: The breakpoint ID for this mapping.
    *     - multiplier: The multiplier for this mapping.
    */
-  public function getKeyedMappings();
+  public function getKeyedMappingDefinitions();
 
   /**
    * Returns the mappings for the responsive image mapping.
@@ -52,7 +52,7 @@ public function getKeyedMappings();
    *   - image_mapping_type
    *   - image_mapping
    */
-  public function getMappings();
+  public function getMappingDefinitions();
 
   /**
    * Sets the breakpoint group for the responsive_image mapping.
@@ -119,13 +119,13 @@ public static function isEmptyMappingDefinition(array $mapping_definition);
    *
    * @return $this
    */
-  public function addMapping($breakpoint_id, $multiplier, array $mapping_definition);
+  public function addMappingDefinition($breakpoint_id, $multiplier, array $mapping_definition);
 
   /**
    * Removes all mappings from the responsive image configuration entity.
    *
    * @return $this
    */
-  public function removeMappings();
+  public function removeMappingDefinitions();
 
 }
diff --git a/core/modules/responsive_image/src/Tests/ResponsiveImageFieldDisplayTest.php b/core/modules/responsive_image/src/Tests/ResponsiveImageFieldDisplayTest.php
index eee1161..281d03e 100644
--- a/core/modules/responsive_image/src/Tests/ResponsiveImageFieldDisplayTest.php
+++ b/core/modules/responsive_image/src/Tests/ResponsiveImageFieldDisplayTest.php
@@ -100,18 +100,18 @@ public function testResponsiveImageFieldFormattersEmptyStyle() {
   protected function addTestMappings($empty_styles = FALSE) {
     if ($empty_styles) {
       $this->responsiveImgMapping
-        ->addMapping('responsive_image_test_module.mobile', '1x', array(
+        ->addMappingDefinition('responsive_image_test_module.mobile', '1x', array(
           'image_mapping_type' => 'image_style',
           'image_mapping' => '',
         ))
-        ->addMapping('responsive_image_test_module.narrow', '1x', array(
+        ->addMappingDefinition('responsive_image_test_module.narrow', '1x', array(
           'image_mapping_type' => 'sizes',
           'image_mapping' => array(
             'sizes' => '(min-width: 700px) 700px, 100vw',
             'sizes_image_styles' => array(),
           ),
         ))
-        ->addMapping('responsive_image_test_module.wide', '1x', array(
+        ->addMappingDefinition('responsive_image_test_module.wide', '1x', array(
           'image_mapping_type' => 'image_style',
           'image_mapping' => '',
         ))
@@ -119,11 +119,11 @@ protected function addTestMappings($empty_styles = FALSE) {
     }
     else {
       $this->responsiveImgMapping
-        ->addMapping('responsive_image_test_module.mobile', '1x', array(
+        ->addMappingDefinition('responsive_image_test_module.mobile', '1x', array(
           'image_mapping_type' => 'image_style',
           'image_mapping' => RESPONSIVE_IMAGE_EMPTY_IMAGE,
         ))
-        ->addMapping('responsive_image_test_module.narrow', '1x', array(
+        ->addMappingDefinition('responsive_image_test_module.narrow', '1x', array(
           'image_mapping_type' => 'sizes',
           'image_mapping' => array(
             'sizes' => '(min-width: 700px) 700px, 100vw',
@@ -133,7 +133,7 @@ protected function addTestMappings($empty_styles = FALSE) {
             ),
           ),
         ))
-        ->addMapping('responsive_image_test_module.wide', '1x', array(
+        ->addMappingDefinition('responsive_image_test_module.wide', '1x', array(
           'image_mapping_type' => 'image_style',
           'image_mapping' => 'large',
         ))
diff --git a/core/modules/responsive_image/templates/responsive-image.html.twig b/core/modules/responsive_image/templates/responsive-image.html.twig
index 6e91316..12501e0 100644
--- a/core/modules/responsive_image/templates/responsive-image.html.twig
+++ b/core/modules/responsive_image/templates/responsive-image.html.twig
@@ -1,7 +1,7 @@
 {#
 /**
  * @file
- * Default theme implementation of a <picture> tag.
+ * Default theme implementation of a responsive image.
  *
  * Available variables:
  * - attributes: HTML attributes for the <picture> tag.
diff --git a/core/modules/responsive_image/tests/src/Unit/ResponsiveImageMappingConfigEntityUnitTest.php b/core/modules/responsive_image/tests/src/Unit/ResponsiveImageMappingConfigEntityUnitTest.php
index 4e59b3c..493ddb5 100644
--- a/core/modules/responsive_image/tests/src/Unit/ResponsiveImageMappingConfigEntityUnitTest.php
+++ b/core/modules/responsive_image/tests/src/Unit/ResponsiveImageMappingConfigEntityUnitTest.php
@@ -79,28 +79,28 @@ public function testCalculateDependencies() {
   }
 
   /**
-   * @covers ::addMapping
-   * @covers ::hasMappings
+   * @covers ::addMappingDefinition
+   * @covers ::hasMappingDefinitions
    */
-  public function testHasMappings() {
+  public function testHasMappingDefinitions() {
     $entity = new ResponsiveImageMapping(array());
-    $this->assertFalse($entity->hasMappings());
-    $entity->addMapping('test_breakpoint', '1x', array(
+    $this->assertFalse($entity->hasMappingDefinitions());
+    $entity->addMappingDefinition('test_breakpoint', '1x', array(
         'image_mapping_type' => 'image_style',
         'image_mapping' => '',
     ));
-    $this->assertFalse($entity->hasMappings());
-    $entity->removeMappings();
-    $entity->addMapping('test_breakpoint', '1x', array(
+    $this->assertFalse($entity->hasMappingDefinitions());
+    $entity->removeMappingDefinitions();
+    $entity->addMappingDefinition('test_breakpoint', '1x', array(
         'image_mapping_type' => 'sizes',
         'image_mapping' => array(
           'sizes' => '(min-width:700px) 700px, 100vw',
           'sizes_image_styles' => array(),
         ),
     ));
-    $this->assertFalse($entity->hasMappings());
-    $entity->removeMappings();
-    $entity->addMapping('test_breakpoint', '1x', array(
+    $this->assertFalse($entity->hasMappingDefinitions());
+    $entity->removeMappingDefinitions();
+    $entity->addMappingDefinition('test_breakpoint', '1x', array(
         'image_mapping_type' => 'sizes',
         'image_mapping' => array(
           'sizes' => '',
@@ -109,15 +109,15 @@ public function testHasMappings() {
           ),
         ),
     ));
-    $this->assertFalse($entity->hasMappings());
-    $entity->removeMappings();
-    $entity->addMapping('test_breakpoint', '1x', array(
+    $this->assertFalse($entity->hasMappingDefinitions());
+    $entity->removeMappingDefinitions();
+    $entity->addMappingDefinition('test_breakpoint', '1x', array(
         'image_mapping_type' => 'image_style',
         'image_mapping' => 'large',
     ));
-    $this->assertTrue($entity->hasMappings());
-    $entity->removeMappings();
-    $entity->addMapping('test_breakpoint', '1x', array(
+    $this->assertTrue($entity->hasMappingDefinitions());
+    $entity->removeMappingDefinitions();
+    $entity->addMappingDefinition('test_breakpoint', '1x', array(
         'image_mapping_type' => 'sizes',
         'image_mapping' => array(
           'sizes' => '(min-width:700px) 700px, 100vw',
@@ -126,16 +126,16 @@ public function testHasMappings() {
           ),
         ),
     ));
-    $this->assertTrue($entity->hasMappings());
+    $this->assertTrue($entity->hasMappingDefinitions());
   }
 
   /**
-   * @covers ::addMapping
+   * @covers ::addMappingDefinition
    * @covers ::getMappingDefinition
    */
   public function testGetMappingDefinition() {
     $entity = new ResponsiveImageMapping(array(''));
-    $entity->addMapping('test_breakpoint', '1x', array(
+    $entity->addMappingDefinition('test_breakpoint', '1x', array(
       'image_mapping_type' => 'image_style',
       'image_mapping' => 'large',
     ));
@@ -150,16 +150,16 @@ public function testGetMappingDefinition() {
   }
 
   /**
-   * @covers ::addMapping
-   * @covers ::getMappings
+   * @covers ::addMappingDefinition
+   * @covers ::getKeyedMappingDefinitions
    */
-  public function testGetKeyedMappings() {
+  public function testGetKeyedMappingDefinitions() {
     $entity = new ResponsiveImageMapping(array(''));
-    $entity->addMapping('test_breakpoint', '1x', array(
+    $entity->addMappingDefinition('test_breakpoint', '1x', array(
       'image_mapping_type' => 'image_style',
       'image_mapping' => 'large',
     ));
-    $entity->addMapping('test_breakpoint', '2x', array(
+    $entity->addMappingDefinition('test_breakpoint', '2x', array(
       'image_mapping_type' => 'sizes',
       'image_mapping' => array(
         'sizes' => '(min-width:700px) 700px, 100vw',
@@ -168,7 +168,7 @@ public function testGetKeyedMappings() {
         ),
       ),
     ));
-    $entity->addMapping('test_breakpoint2', '1x',  array(
+    $entity->addMappingDefinition('test_breakpoint2', '1x',  array(
       'image_mapping_type' => 'image_style',
       'image_mapping' => 'thumbnail',
     ));
@@ -202,10 +202,10 @@ public function testGetKeyedMappings() {
         ),
       )
     );
-    $this->assertEquals($expected, $entity->getKeyedMappings());
+    $this->assertEquals($expected, $entity->getKeyedMappingDefinitions());
 
     // Add another mapping to ensure keyed mapping static cache is rebuilt.
-    $entity->addMapping('test_breakpoint2', '2x', array(
+    $entity->addMappingDefinition('test_breakpoint2', '2x', array(
       'image_mapping_type' => 'image_style',
       'image_mapping' => 'medium',
     ));
@@ -215,20 +215,20 @@ public function testGetKeyedMappings() {
       'image_mapping_type' => 'image_style',
       'image_mapping' => 'medium',
     );
-    $this->assertEquals($expected, $entity->getKeyedMappings());
+    $this->assertEquals($expected, $entity->getKeyedMappingDefinitions());
   }
 
   /**
-   * @covers ::addMapping
-   * @covers ::getMappings
+   * @covers ::addMappingDefinition
+   * @covers ::getMappingDefinitions
    */
-  public function testGetMappings() {
+  public function testGetMappingDefinitions() {
     $entity = new ResponsiveImageMapping(array(''));
-    $entity->addMapping('test_breakpoint', '1x', array(
+    $entity->addMappingDefinition('test_breakpoint', '1x', array(
       'image_mapping_type' => 'image_style',
       'image_mapping' => 'large',
     ));
-    $entity->addMapping('test_breakpoint', '2x', array(
+    $entity->addMappingDefinition('test_breakpoint', '2x', array(
       'image_mapping_type' => 'sizes',
       'image_mapping' => array(
         'sizes' => '(min-width:700px) 700px, 100vw',
@@ -237,7 +237,7 @@ public function testGetMappings() {
         ),
       ),
     ));
-    $entity->addMapping('test_breakpoint2', '1x',  array(
+    $entity->addMappingDefinition('test_breakpoint2', '1x',  array(
       'image_mapping_type' => 'image_style',
       'image_mapping' => 'thumbnail',
     ));
@@ -267,20 +267,20 @@ public function testGetMappings() {
         'image_mapping' => 'thumbnail',
       ),
     );
-    $this->assertEquals($expected, $entity->getMappings());
+    $this->assertEquals($expected, $entity->getMappingDefinitions());
   }
 
   /**
-   * @covers ::addMapping
-   * @covers ::removeMappings
+   * @covers ::addMappingDefinition
+   * @covers ::removeMappingDefinitions
    */
-  public function testRemoveMappings() {
+  public function testRemoveMappingDefinitions() {
     $entity = new ResponsiveImageMapping(array(''));
-    $entity->addMapping('test_breakpoint', '1x', array(
+    $entity->addMappingDefinition('test_breakpoint', '1x', array(
       'image_mapping_type' => 'image_style',
       'image_mapping' => 'large',
     ));
-    $entity->addMapping('test_breakpoint', '2x', array(
+    $entity->addMappingDefinition('test_breakpoint', '2x', array(
       'image_mapping_type' => 'sizes',
       'image_mapping' => array(
         'sizes' => '(min-width:700px) 700px, 100vw',
@@ -289,16 +289,16 @@ public function testRemoveMappings() {
         ),
       ),
     ));
-    $entity->addMapping('test_breakpoint2', '1x',  array(
+    $entity->addMappingDefinition('test_breakpoint2', '1x',  array(
       'image_mapping_type' => 'image_style',
       'image_mapping' => 'thumbnail',
     ));
 
-    $this->assertTrue($entity->hasMappings());
-    $entity->removeMappings();
-    $this->assertEmpty($entity->getMappings());
-    $this->assertEmpty($entity->getKeyedMappings());
-    $this->assertFalse($entity->hasMappings());
+    $this->assertTrue($entity->hasMappingDefinitions());
+    $entity->removeMappingDefinitions();
+    $this->assertEmpty($entity->getMappingDefinitions());
+    $this->assertEmpty($entity->getKeyedMappingDefinitions());
+    $this->assertFalse($entity->hasMappingDefinitions());
   }
 
   /**
@@ -307,11 +307,11 @@ public function testRemoveMappings() {
    */
   public function testSetBreakpointGroup() {
     $entity = new ResponsiveImageMapping(array('breakpointGroup' => 'test_group'));
-    $entity->addMapping('test_breakpoint', '1x', array(
+    $entity->addMappingDefinition('test_breakpoint', '1x', array(
       'image_mapping_type' => 'image_style',
       'image_mapping' => 'large',
     ));
-    $entity->addMapping('test_breakpoint', '2x', array(
+    $entity->addMappingDefinition('test_breakpoint', '2x', array(
       'image_mapping_type' => 'sizes',
       'image_mapping' => array(
         'sizes' => '(min-width:700px) 700px, 100vw',
@@ -320,20 +320,20 @@ public function testSetBreakpointGroup() {
         ),
       ),
     ));
-    $entity->addMapping('test_breakpoint2', '1x',  array(
+    $entity->addMappingDefinition('test_breakpoint2', '1x',  array(
       'image_mapping_type' => 'image_style',
       'image_mapping' => 'thumbnail',
     ));
 
     // Ensure that setting to same group does not remove mappings.
     $entity->setBreakpointGroup('test_group');
-    $this->assertTrue($entity->hasMappings());
+    $this->assertTrue($entity->hasMappingDefinitions());
     $this->assertEquals('test_group', $entity->getBreakpointGroup());
 
     // Ensure that changing the group removes mappings.
     $entity->setBreakpointGroup('test_group2');
     $this->assertEquals('test_group2', $entity->getBreakpointGroup());
-    $this->assertFalse($entity->hasMappings());
+    $this->assertFalse($entity->hasMappingDefinitions());
   }
 
 }