That part of the helmet that defends the sight. This is a the common definition found in heraldry books, and it is wrong.  The “bavier” or “beaver” in English, is actually the chin guard.  After European colonization of the New World the small fur-bearing mammal known as the Beaver began to appear on Coats of Arms — symbolizing industrious adventures in the New World.  Be careful of confusion of these two terms.