The Collaborative International Dictionary of English v.0.48:

Pileus \Pi"le*us\, n.; pl. Pilei. [L., a felt cap.]
   1. (Rom. Antiq.) A kind of skull cap of felt.
      [1913 Webster]

   2. (Bot.) The expanded upper portion of many of the fungi.
      See Mushroom.
      [1913 Webster]

   3. (Zool.) The top of the head of a bird, from the bill to
      the nape.
      [1913 Webster]

WordNet (r) 3.0 (2006):

pileus
    n 1: a fruiting structure resembling an umbrella or a cone that
         forms the top of a stalked fleshy fungus such as a mushroom
         [syn: cap, pileus]