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

Moby Thesaurus II by Grady Ward, 1.0:

163 Moby Thesaurus words for "metaphor":
   accordance, agent, agreement, alikeness, allegory, alliance,
   alliteration, allusion, alternate, alternative, anacoluthon,
   anadiplosis, analogue, analogy, anaphora, anastrophe, antiphrasis,
   antithesis, antonomasia, aping, apophasis, aporia, aposiopesis,
   apostrophe, approach, approximation, assimilation, backup,
   balancing, catachresis, change, changeling, chiasmus,
   circumlocution, climax, closeness, community, comparability,
   comparative anatomy, comparative degree, comparative grammar,
   comparative judgment, comparative linguistics,
   comparative literature, comparative method, compare, comparing,
   comparison, conformity, confrontation, confrontment, contrast,
   contrastiveness, conversion, copy, copying, correlation,
   correspondence, counterfeit, deputy, distinction, distinctiveness,
   double, dummy, ecphonesis, emphasis, equal, equivalent, ersatz,
   exchange, exclamation, fake, figure of speech, fill-in, gemination,
   ghost, ghostwriter, hypallage, hyperbaton, hyperbole, identity,
   image, imagery, imitation, inversion, irony, likeness, likening,
   litotes, locum tenens, makeshift, malapropism, matching, meiosis,
   metonymy, mimicking, nearness, next best thing, onomatopoeia,
   opposing, opposition, oxymoron, parallelism, paregmenon,
   parenthesis, parity, periphrasis, personification, personnel,
   phony, pinch hitter, pleonasm, preterition, prolepsis, proportion,
   proxy, reference, regression, relation, relief, repetition,
   replacement, representative, resemblance, reserves, ringer,
   sameness, sarcasm, second string, secondary, semblance, sign,
   similarity, simile, similitude, simulation, spares, spoonerism,
   stand-in, sub, substituent, substitute, substitution, succedaneum,
   superseder, supplanter, surrogate, syllepsis, symbol, symbolism,
   symploce, synecdoche, third string, token, trope,
   trope of comparison, understudy, utility player, vicar,
   vice-president, vice-regent, weighing, zeugma