You can use any name for the function.
function custom_postype(){
register_post_type('cus_post',array(
'labels'=>array(
'name'=>'khaiyam'// Use any name you want to show in menu for your users
),
'public'=>true,// **Must required
'supports'=>array('title','editor','thumbnail')// Features you want to provide on your posts
));
}
add_action('after_setup_theme','custom_postytpe');
or
add_action('init','custom_postytpe');
You can use any of the hooks you want but of course they have different meaning and uses.