aventail

A curtain or flap of mail, fastened to a helmet, or to a coif (hood) of mail, covering the lower face, neck, and shoulders.

[…] the hood was increasingly replaced with a mail curtain (the camail or aventail) suspended from the outside of the bascinet, and the bascinet thus augmented gradually replaced the clumsy great helm as the principal defense […]

Synonym of ventail (“movable (solid plate) front to a helmet”).