The Collaborative International Dictionary of English v.0.48:

Sea star \Sea" star`\ (Zool.)
   A starfish, or brittle star.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Starfish \Star"fish`\ (-f[i^]sh`), n.
   1. (Zool.) Any one of numerous species of echinoderms
      belonging to the class Asterioidea, in which the body is
      star-shaped and usually has five rays, though the number
      of rays varies from five to forty or more. The rays are
      often long, but are sometimes so short as to appear only
      as angles to the disklike body. Called also sea star,
      five-finger, and stellerid.
      [1913 Webster]

   Note: The ophiuroids are also sometimes called starfishes.
         See Brittle star, and Ophiuroidea.
         [1913 Webster]

   2. (Zool.) The dollar fish, or butterfish.
      [1913 Webster]

WordNet (r) 3.0 (2006):

sea star
    n 1: echinoderms characterized by five arms extending from a
         central disk [syn: starfish, sea star]