(historical) Synonym of aventail (“mail curtain or flap, on a helmet or a mail coif, that protects the lower face and neck”).
(historical) The movable front part of a medieval helmet, originally including the visor but later specifically the separate lower section.
The helmet, in its improved state, was composed of two parts; the headpiece, which was strengthened within by several circles of iron; and the visor or ventail, which (as the names imply) was a sort of grating to see or breathe through, so contrived as by sliding in a groove, or turning on a pivot, to be raised or lowered […]
no ventaille to this darkening / and masked face lifts to grope the air […].
(historical, rare) A vent or breathing-hole in a medieval helmet, for the admission of air.
398. ventayles : a pun on ventails, 'something acting as a sail or fan', and ventails, the air holes or vents in the vizor of a helmet.