The Collaborative International Dictionary of English v.0.48:

orc \orc\ ([^o]rk), n. [L. orca, a kind of whale: cf. F. orque.]
   1. (Zool.) Any of several cetaceans, especialy the grampus
      (Grampus griseus) of the dolphin family. [Written also
      ork and orch.] --Milton.
      [1913 Webster]

            An island salt and bare,
            The haunt of seals, and orcs, and sea-mews' clang.
                                                  --Milton (Par.
                                                  Lost xi. 835).

   2. (Mythology) A mythical monster of varying descriptions; an
      ogre.
      [PJC]

            Goblins, hobgoblins, and orcs of the worst
            description.                          --J. J.
                                                  Tolkien (The
                                                  Hobbit)

   3. The orca.
      [PJC]

WordNet (r) 3.0 (2006):

Grampus griseus
    n 1: slaty-grey blunt-nosed dolphin common in northern seas
         [syn: grampus, Grampus griseus]