Boss

From MiddleWiki

A Boss is a round "bowl" of metal used in armor (most often on a shield as in a center boss shield which has a round boss in the center to accomodate the handle and protect the hand of the warrior carrying it.)

Bosses also appear in bookbinding where small round brass or other metal bosses are attached to the corners and/or center of a book to protect the book by keeping it up off of the table upon which it is set.