The Collaborative International Dictionary of English v.0.48:
Hydroid \Hy"droid\, a. [Hydra + -oid.] (Zool.) Related to, or resembling, the hydra; of or pertaining to the Hydroidea. -- n. One of the Hydroideas. [1913 Webster]WordNet (r) 3.0 (2006):
hydroid n 1: colonial coelenterates having the polyp phase dominant [syn: hydrozoan, hydroid]