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.]
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            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.
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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.
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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]