The Collaborative International Dictionary of English v.0.48:

Defeat \De*feat"\, n. [Cf. F. d['e]faite, fr. d['e]faire. See
   Defeat, v.]
   1. An undoing or annulling; destruction. [Obs.]
      [1913 Webster]

            Upon whose property and most dear life
            A damned defeat was made.             --Shak.
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   2. Frustration by rendering null and void, or by prevention
      of success; as, the defeat of a plan or design.
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   3. An overthrow, as of an army in battle; loss of a battle;
      repulse suffered; discomfiture; -- opposed to victory.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Defeat \De*feat"\, v. t. [imp. & p. p. Defeated; p. pr. & vb.
   n. Defeating.] [From F. d['e]fait, OF. desfait, p. p. ofe
   d['e]faire, OF. desfaire, to undo; L. dis- + facere to do.
   See Feat, Fact, and cf. Disfashion.]
   1. To undo; to disfigure; to destroy. [Obs.]
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            His unkindness may defeat my life.    --Shak.
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   2. To render null and void, as a title; to frustrate, as
      hope; to deprive, as of an estate.
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            He finds himself naturally to dread a superior Being
            that can defeat all his designs, and disappoint all
            his hopes.                            --Tillotson.
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            The escheators . . . defeated the right heir of his
            succession.                           --Hallam.
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            In one instance he defeated his own purpose. --A. W.
                                                  Ward.
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   3. To overcome or vanquish, as an army; to check, disperse,
      or ruin by victory; to overthrow.
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   4. To resist with success; as, to defeat an assault.
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            Sharp reasons to defeat the law.      --Shak.

   Syn: To baffle; disappoint; frustrate.
        [1913 Webster]

WordNet (r) 3.0 (2006):

defeat
    n 1: an unsuccessful ending to a struggle or contest; "it was a
         narrow defeat"; "the army's only defeat"; "they suffered a
         convincing licking" [syn: defeat, licking] [ant:
         triumph, victory]
    2: the feeling that accompanies an experience of being thwarted
       in attaining your goals [syn: frustration, defeat]
    v 1: win a victory over; "You must overcome all difficulties";
         "defeat your enemies"; "He overcame his shyness"; "He
         overcame his infirmity"; "Her anger got the better of her
         and she blew up" [syn: get the better of, overcome,
         defeat]
    2: thwart the passage of; "kill a motion"; "he shot down the
       student's proposal" [syn: kill, shoot down, defeat,
       vote down, vote out]

Moby Thesaurus II by Grady Ward, 1.0:

212 Moby Thesaurus words for "defeat":
   KO, Waterloo, answer, answer conclusively, argue down, baffle,
   bafflement, balk, balking, bankruptcy, bar, bear down,
   bear the palm, beat, beat all hollow, beat down, beat hollow,
   beating, best, betrayed hope, bilk, blast, blasted expectation,
   blighted hope, block, blow, brave, bring down, buffet, cast down,
   challenge, check, checkmate, circumvent, cleaning, cleanup,
   clobber, clobbering, comedown, confound, confounding, confront,
   confute, conquer, conquest, contradict, contravene, controvert,
   cook, counter, counteract, countermand, counterwork, cross,
   cruel disappointment, crush, dash, dashed hope, debacle,
   defeasance, defeat expectation, defy, demolish, deny, destroy,
   disappoint, disappointment, discomfit, discomfiture, disconcert,
   disconcertion, discountenance, dish, disillusion, disillusionment,
   dismiss, dispose of, disrupt, dissatisfaction, dissatisfy, do for,
   do in, downcast, downthrow, drub, drubbing, dusting, elude, end,
   failure, fallen countenance, fiasco, finish, fix, fizzle, floor,
   flummox, foil, foiling, forlorn hope, frustrate, frustration,
   futility, hide, hinder, hope deferred, hors de combat, ill success,
   impede, knock out, knock the chocks, lambaste, lambasting, lather,
   let down, letdown, lick, licking, losing game, mirage, miscarriage,
   no go, nonaccomplishment, nonplus, nonsuccess, obstruct, outclass,
   outdo, outfight, outgeneral, outmaneuver, outpoint, outrun,
   outsail, outshine, overcome, overpower, overthrow, overturn,
   overwhelm, parry, perplex, prevail over, put, put to silence,
   rebuff, rebut, reduce, reduce to silence, refute, repress, repulse,
   reversal, reverse, rise above, rout, ruin, sabotage, scotch,
   scuttle, setback, settle, shellacking, shoot down, shut up,
   silence, sink, skin, skin alive, smash all opposition,
   sore disappointment, spike, spoil, squash, squelch, stonewall,
   stop, stump, subdue, subjugate, subvert, successlessness, suppress,
   surmount, take the cake, tantalization, tantalize, tease,
   terminate, thrash, thrashing, thwart, thwarting, torpedo, trim,
   triumph, triumph over, trounce, trouncing, undermine, undo,
   undoing, unsuccess, unsuccessfulness, upset, uselessness, vanquish,
   vanquishment, warming, whip, win, worst