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.
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   2. (Bot.) The expanded upper portion of many of the fungi.
      See Mushroom.
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   3. (Zool.) The top of the head of a bird, from the bill to
      the nape.
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