WordPress at a glance
action is not described

wp action-hook . WP 2.1.0

Fires once the WordPress environment has been set up.

Usage

add_action( 'wp', 'action_function_name_455' );
function action_function_name_455( $this ){
	// action...
}
$this(WP)
Current WordPress environment instance (passed by reference).

Where the hook is called

WP::main()
wp-includes/class-wp.php 728
do_action_ref_array( 'wp', array( &$this ) );

Where the hook is used (in WP core)

wp-includes/class-wp-customize-widgets.php 357
add_action( 'wp', array( $this, 'customize_register' ) );