Source : Webster's Revised Unabridged Dictionary (1913)

Metaphor \Met"a*phor\, n. [F. m['e]taphore, L. metaphora, fr.
   Gr. ?, fr. ? 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.

   Note: The statement, ``that man is a fox,'' is a metaphor;
         but ``that man is like a fox,'' is a simile,
         similitude, or comparison.

Source : WordNet®

metaphor
     n : 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