Source : 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. (Zo["o]l.) The top of the head of a bird, from the bill to
      the nape.

Source : WordNet®

pileus
     n : a fruiting structure resembling an umbrella that forms the
         top of a stalked fleshy fungus such as a mushroom [syn: {cap}]
     [also: {pilei} (pl)]