Webster’s Revised Unabridged Dictionary · 1913
Pileus
Pi′le·us, n.; pl. Pilei (#). [L., a felt cap.] 1. (Rom. Antiq.) A kind of skull cap of felt.
2. (Bot.) The expanded upper portion of many of the fungi. See Mushroom.
3. (Zoöl.) The top of the head of a bird, from the bill to the nape.