The Collaborative International Dictionary of English v.0.48:

Ventail \Vent"ail\ (v[e^]nt"[=a]l), n. [OF. ventaille, F.
   ventail. See Ventilate, and cf. Aventail.]
   That part of a helmet which is intended for the admission of
   air, -- sometimes in the visor. --Spenser.
   [1913 Webster]

         Her ventail up so high that he descried
         Her goodly visage and her beauty's pride. --Fairfax.
   [1913 Webster]

WordNet (r) 3.0 (2006):

ventail
    n 1: a medieval hood of mail suspended from a basinet to protect
         the head and neck [syn: camail, aventail, ventail]