ventail
The movable front part of a medieval helmet, originally including the visor but later specifically the separate lower section.
historical
A comely knight, all arm'd in complete wize, / Through whose bright ventayle lifted vp on hye / His manly face [...] Lookt foorth [...].
A vent or breathing-hole in a medieval helmet, for the admission of air.
historical, rare
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.
Where it comes from
From Middle English ventaile (“mail over lower face and neck; lower front piece of helmet; air hole in helmet”), from Old French ventaille (“lower opening in helmet for air”). Related to aventail.
Adapted from the Wiktionary entry ventail (CC BY-SA 4.0).