The Collaborative International Dictionary of English v.0.48:

Vindicate \Vin"di*cate\, v. t. [imp. & p. p. Vindicated; p.
   pr. & vb. n. Vindicating.] [L. vindicatus, p. p. of
   vindicare to lay claim to, defend, avenge. See Vengeance.]
   1. To lay claim to; to assert a right to; to claim. [R.]
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            Is thine alone the seed that strews the plain?
            The birds of heaven shall vindicate their grain.
                                                  --Pope.
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   2. To maintain or defend with success; to prove to be valid;
      to assert convincingly; to sustain against assault; as, to
      vindicate a right, claim, or title.
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   3. To support or maintain as true or correct, against denial,
      censure, or objections; to defend; to justify.
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            When the respondent denies any proposition, the
            opponent must directly vindicate . . . that
            proposition.                          --I. Watts.
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            Laugh where we must, be candid where we can,
            But vindicate the ways of God to man. --Pope.
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   4. To maintain, as a law or a cause, by overthrowing enemies.
      --Milton.
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   5. To liberate; to set free; to deliver. [Obs.]
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            I am confident he deserves much more
            That vindicates his country from a tyrant
            Than he that saves a citizen.         --Massinger.
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   6. To avenge; to punish; as, a war to vindicate or punish
      infidelity. [Obs.] --Bacon.
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            God is more powerful to exact subjection and to
            vindicate rebellion.                  --Bp. Pearson.
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   Syn: To assert; maintain; claim. See Assert.
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WordNet (r) 3.0 (2006):

vindicate
    v 1: show to be right by providing justification or proof;
         "vindicate a claim" [syn: justify, vindicate]
    2: maintain, uphold, or defend; "vindicate the rights of the
       citizens"
    3: clear of accusation, blame, suspicion, or doubt with
       supporting proof; "You must vindicate yourself and fight this
       libel"