The Collaborative International Dictionary of English v.0.48:

Hippocampus \Hip`po*cam"pus\, n. [L., the sea horse, Gr. ? a
   hippocampus (in senses 1 and 2); "i`ppos horse + ? to bend.]
   1. (Class. Myth.) A fabulous monster, with the head and fore
      quarters of a horse joined to the tail of a dolphin or
      other fish (Hippocampus brevirostris), -- seen in
      Pompeian paintings, attached to the chariot of Neptune.
      --Fairholt.
      [1913 Webster]

   2. (Zool.) A genus of lophobranch fishes of several species
      in which the head and neck have some resemblance to those
      of a horse; -- called also sea horse.
      [1913 Webster]

   Note: They swim slowly, in an erect position, and often cling
         to seaweeds by means of the incurved prehensile tail.
         The male has a ventral pouch, in which it carries the
         eggs till hatched.
         [1913 Webster]

   3. (Zool.) A name applied to either of two ridges of white
      matter in each lateral ventricle of the brain. The larger
      is called hippocampus major or simply hippocampus. The
      smaller, hippocampus minor, is called also ergot and
      calcar.
      [1913 Webster]

WordNet (r) 3.0 (2006):

hippocampus
    n 1: a complex neural structure (shaped like a sea horse)
         consisting of grey matter and located on the floor of each
         lateral ventricle; intimately involved in motivation and
         emotion as part of the limbic system; has a central role in
         the formation of memories
    2: seahorses [syn: Hippocampus, genus Hippocampus]