Source : Webster's Revised Unabridged Dictionary (1913)

Hydroid \Hy"droid\, a. [Hydra + -oid.] (Zo["o]l.)
   Related to, or resembling, the hydra; of or pertaining to the
   Hydroidea. -- n. One of the Hydroideas.

Source : WordNet®

hydroid
     n : colonial coelenterates having the polyp phase dominant [syn:
          {hydrozoan}]