Definition of ventails:
(n) :
(historical) The movable front part of a medieval helmet, originally including the visor but later specifically the separate lower section.
(n) :
(historical) Synonym of aventail (“mail curtain or flap, on a helmet or a mail coif, that protects the lower face and neck”).
(n) :
(historical, rare) A vent or breathing-hole in a medieval helmet, for the admission of air.