WordPress Codex: register_nav_menu()

register_nav_menu() is a WP function that registers a single custom navigation menu in the new custom menu editor of WordPress 3.0. This allows for creation of custom menus in the dashboard for use in your theme.

See register_nav_menus() for creating multiple menus at once.

Usage
register_nav_menu( $location, $description );
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Parameters
$location
(string) (required) Menu
location identifier, like a slug.
Default: None

$description
(string) (required) The default value to
return if no value is returned
(ie. the option is not in the database).
Default: None

Return values
None.

Examples
register_nav_menu( 'primary', 'Primary Menu' );
?>

More information: http://codex.wordpress.org/Function_Reference/register_nav_menu