The Collaborative International Dictionary of English v.0.48:

metaphor \met"a*phor`\ (m[e^]t"[.a]*f[^o]r` or
   m[e^]t"[.a]*f[~e]r), n. [F. m['e]taphore, L. metaphora, fr.
   Gr. metafora`, fr. metafe`rein to carry over, transfer; meta`
   beyond, over + fe`rein to bring, bear.] (Rhet.)
   The transference of the relation between one set of objects
   to another set for the purpose of brief explanation; a
   compressed simile; e. g., the ship plows the sea. --Abbott &
   Seeley. "All the world's a stage." --Shak.
   [1913 Webster]

   Note: The statement, "that man is a fox," is a metaphor; but
         "that man is like a fox," is a simile, similitude, or
         comparison.
         [1913 Webster] Metaphoric

WordNet (r) 3.0 (2006):

metaphor
    n 1: a figure of speech in which an expression is used to refer
         to something that it does not literally denote in order to
         suggest a similarity