The Collaborative International Dictionary of English v.0.48:

hen-of-the-woods \hen-of-the-woods\ n.
   A large grayish-brown edible fungus (Polyporus frondosus)
   forming a mass of overlapping caps at the base of trees that
   somewhat resembles a hen.

   Syn: hen of the woods, Polyporus frondosus.
        [WordNet 1.5]

WordNet (r) 3.0 (2006):

Polyporus frondosus
    n 1: large greyish-brown edible fungus forming a mass of
         overlapping caps that somewhat resembles a hen at the base
         of trees [syn: hen-of-the-woods, hen of the woods,
         Polyporus frondosus, Grifola frondosa]