The Collaborative International Dictionary of English v.0.48:

dolphin \dol"phin\ (d[o^]l"f[i^]n), n. [F. dauphin dolphin,
   dauphin, earlier spelt also doffin; cf. OF. dalphinal of the
   dauphin; fr. L. delphinus, Gr. delfi`s a dolphin (in senses
   1, 2, 3, & 6), perh. properly, belly fish; cf. delfy`s womb,
   Skr. garbha; perh. akin to E. calf. Cf. Dauphin,
   Delphine.]
   1. (Zool.) A cetacean of the genus Delphinus and allied
      genera (esp. Delphinus delphis); the true dolphin.
      [1913 Webster]

   Note: The dolphin of the ancients (Delphinus delphis) is
         common in the Mediterranean and Atlantic, and attains a
         length of from six to eight feet.
         [1913 Webster]

   2. (Zool.) The Coryph[ae]na hippuris, a fish of about five
      feet in length, celebrated for its surprising changes of
      color when dying. It is the fish commonly known as the
      dolphin. The term is also applied to the related
      Coryphaena equisetis. Called also dolphinfish and
      (especially in Hawaii) mahimahi. See also dolphinfish
      and Coryph[ae]noid.

   Syn: dolphinfish, mahimahi.
        [1913 Webster +PJC]

   3. [Gr. delfi`s] (Gr. Antiq.) A mass of iron or lead hung
      from the yardarm, in readiness to be dropped on the deck
      of an enemy's vessel.
      [1913 Webster]

   4. (Naut.)
      (a) A kind of wreath or strap of plaited cordage.
      (b) A spar or buoy held by an anchor and furnished with a
          ring to which ships may fasten their cables. --R. H.
          Dana.
      (c) A mooring post on a wharf or beach.
      (d) A permanent fender around a heavy boat just below the
          gunwale. --Ham. Nav. Encyc.
          [1913 Webster]

   5. (Gun.) In old ordnance, one of the handles above the
      trunnions by which a cannon was lifted.
      [1913 Webster]

   6. (Astron.) A small constellation between Aquila and
      Pegasus. See Delphinus, n., 2.
      [1913 Webster]

   Dolphin fly (Zool.), the black, bean, or collier, Aphis
      (Aphis fable), destructive to beans.

   Dolphin striker (Naut.), a short vertical spar under the
      bowsprit.
      [1913 Webster]

WordNet (r) 3.0 (2006):

dolphin striker
    n 1: spar under the bowsprit of a sailboat [syn: dolphin
         striker, martingale]