The Collaborative International Dictionary of English v.0.48:

Polyp \Pol"yp\, n. [L. polypus, Gr. ?, ?, literally,
   many-footed; poly`s many + ?, ?, foot: cf. F. polype. See
   Poly- and Foot, and cf. Polypode, Polypody, Poulp.]
   (Zool.)
   (a) One of the feeding or nutritive zooids of a hydroid or
       coral.
   (b) One of the Anthozoa.
   (c) pl. Same as Anthozoa. See Anthozoa, Madreporaria,
       Hydroid. [Written also polype.]
       [1913 Webster]

   Fresh-water polyp, the hydra.

   Polyp stem (Zool.), that portion of the stem of a
      siphonophore which bears the polypites, or feeding zooids.
      [1913 Webster]

WordNet (r) 3.0 (2006):

polyp
    n 1: a small vascular growth on the surface of a mucous membrane
         [syn: polyp, polypus]
    2: one of two forms that coelenterates take (e.g. a hydra or
       coral): usually sedentary with a hollow cylindrical body
       usually with a ring of tentacles around the mouth; "in some
       species of coelenterate, polyps are a phase in the life cycle
       that alternates with a medusoid phase"