intermediate_image_sizes

/**
 * Get the available image sizes
 * @since 3.0.0
 * @return array Returns a filtered array of image size strings
 */
function get_intermediate_image_sizes() {
        global $_wp_additional_image_sizes;
        $image_sizes = array('thumbnail', 'medium', 'large'); // Standard sizes
        if ( isset( $_wp_additional_image_sizes ) && count( $_wp_additional_image_sizes ) )
                $image_sizes = array_merge( $image_sizes, array_keys( $_wp_additional_image_sizes ) );

        /**
         * Filter the list of intermediate image sizes.
         *
         * @since 2.5.0
         *
         * @param array $image_sizes An array of intermediate image sizes. Defaults
         *                           are 'thumbnail', 'medium', 'large'.
         */
        return [apply_filters](http://codex.wordpress.org/Function_Reference/apply_filters)( '[intermediate_image_sizes](http://codex.wordpress.org/Plugin_API/Filter_Reference/intermediate_image_sizes)', $image_sizes );
}