WordPress at a glance
action is not described

admin_print_styles action-hook . WP 1.0

This action is documented in wp-admin/admin-header.php

Usage

add_action( 'admin_print_styles', 'action_function_name_8006' );
function action_function_name_8006(){
	// action...
}

Where the hook is called

WP_Customize_Widgets::print_styles()
admin_print_styles
iframe_header()
admin_print_styles
wp_iframe()
admin_print_styles
wp-includes/class-wp-customize-widgets.php 647
do_action( 'admin_print_styles' );
wp-admin/includes/template.php 2046
do_action( 'admin_print_styles' );
wp-admin/includes/media.php 550
do_action( 'admin_print_styles' );
wp-admin/admin-header.php 116
do_action( 'admin_print_styles' );

Where in WP core the hook is used WordPress

wp-includes/class-wp-customize-widgets.php 52
add_filter( 'admin_print_styles', 'wp_resource_hints', 1 );
wp-includes/class-wp-customize-widgets.php 357
add_action( 'admin_print_styles', 'print_emoji_styles' );
wp-includes/class-wp-customize-widgets.php 358
add_action( 'admin_print_styles', 'print_admin_styles', 20 );