WordPress at a glance
action is not described

rest_api_init action-hook . WP 4.4.0

Fires when preparing to serve an API request.

Endpoint objects should be created and register their hooks on this action rather than another action to ensure they're only loaded when needed.

Usage

add_action( 'rest_api_init', 'action_function_name_9953' );
function action_function_name_9953( $wp_rest_server ){
	// action...
}
$wp_rest_server(WP_REST_Server)
Server object.

Where the hook is called

rest_get_server()
WC_CLI_Runner::after_wp_load()
wp-includes/rest-api.php 461
do_action( 'rest_api_init', $wp_rest_server );
woocommerce/includes/cli/class-wc-cli-runner.php 59
do_action( 'rest_api_init', $wp_rest_server );

Where the hook is used (in WP core)

wp-includes/default-filters.php 433
add_action( 'rest_api_init', 'rest_api_default_filters', 10, 1 );
wp-includes/default-filters.php 434
add_action( 'rest_api_init', 'register_initial_settings', 10 );
wp-includes/default-filters.php 435
add_action( 'rest_api_init', 'create_initial_rest_routes', 99 );
wp-includes/default-filters.php 531
add_action( 'rest_api_init', 'wp_oembed_register_route' );