The Collaborative International Dictionary of English v.0.48:

acknowledge \ac*knowl"edge\ ([a^]k*n[o^]l"[e^]j), v. t. [imp. &
   p. p. acknowledged ([a^]k*n[o^]l"[e^]jd); p. pr. & vb. n.
   acknowledging ([a^]k*n[o^]l"[e^]j*[i^]ng).] [Prob. fr.
   pref. a- + the verb knowledge. See Knowledge, and cf.
   Acknow.]
   1. To own or admit the knowledge of; to recognize as a fact
      or truth; to declare one's belief in; as, to acknowledge
      the being of a God.
      [1913 Webster]

            I acknowledge my transgressions.      --Ps. li. 3.
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            For ends generally acknowledged to be good.
                                                  --Macaulay.
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   2. To own or recognize in a particular character or
      relationship; to admit the claims or authority of; to give
      recognition to.
      [1913 Webster]

            In all thy ways acknowledge Him.      --Prov. iii.
                                                  6.
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            By my soul, I'll ne'er acknowledge thee. --Shak.
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   3. To own with gratitude or as a benefit or an obligation;
      as, to acknowledge a favor, the receipt of a letter.
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            They his gifts acknowledged none.     --Milton.
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   4. To own as genuine; to assent to, as a legal instrument, to
      give it validity; to avow or admit in legal form; as, to
      acknowledge a deed.
      [1913 Webster]

   Syn: To avow; proclaim; recognize; own; admit; allow;
        concede; confess.

   Usage: Acknowledge, Recognize. Acknowledge is opposed to
          keep back, or conceal, and supposes that something had
          been previously known to us (though perhaps not to
          others) which we now feel bound to lay open or make
          public. Thus, a man acknowledges a secret marriage;
          one who has done wrong acknowledges his fault; and
          author acknowledges his obligation to those who have
          aided him; we acknowledge our ignorance. Recognize
          supposes that we have either forgotten or not had the
          evidence of a thing distinctly before our minds, but
          that now we know it (as it were) anew, or receive and
          admit in on the ground of the evidence it brings.
          Thus, we recognize a friend after a long absence. We
          recognize facts, principles, truths, etc., when their
          evidence is brought up fresh to the mind; as, bad men
          usually recognize the providence of God in seasons of
          danger. A foreign minister, consul, or agent, of any
          kind, is recognized on the ground of his producing
          satisfactory credentials. See also Confess.
          [1913 Webster]

WordNet (r) 3.0 (2006):

acknowledge
    v 1: declare to be true or admit the existence or reality or
         truth of; "He admitted his errors"; "She acknowledged that
         she might have forgotten" [syn: admit, acknowledge]
         [ant: deny]
    2: report the receipt of; "The program committee acknowledged
       the submission of the authors of the paper" [syn:
       acknowledge, receipt]
    3: express recognition of the presence or existence of, or
       acquaintance with; "He never acknowledges his colleagues when
       they run into him in the hallway"; "She acknowledged his
       complement with a smile"; "it is important to acknowledge the
       work of others in one's own writing" [syn: notice,
       acknowledge]
    4: express obligation, thanks, or gratitude for; "We must
       acknowledge the kindness she showed towards us" [syn:
       acknowledge, recognize, recognise]
    5: accept as legally binding and valid; "acknowledge the deed"
    6: accept (someone) to be what is claimed or accept his power
       and authority; "The Crown Prince was acknowledged as the true
       heir to the throne"; "We do not recognize your gods" [syn:
       acknowledge, recognize, recognise, know]

Moby Thesaurus II by Grady Ward, 1.0:

137 Moby Thesaurus words for "acknowledge":
   accept, account for, accredit with, accrete to, admit,
   admit everything, affirm, agree, agree provisionally, allege,
   allow, announce, answer, answer back, apply to, ascribe to,
   assent grudgingly, asseverate, assign to, attach to, attest,
   attribute to, aver, avouch, avow, bear the expense, bear witness,
   blame, blame for, blame on, bless, bring home to, certify,
   charge on, charge to, chip in, come back, come back at, come clean,
   concede, confess, connect with, consider, cop a plea, credit,
   credit with, declare, deem, defray, defray expenses, depone,
   depose, disclose, divulge, echo, express general agreement,
   fasten upon, father upon, finance, fix on, fix upon, flash back,
   foot the bill, fund, give acknowledgment, give answer, give credit,
   give evidence, give thanks, go Dutch, go along with, grant,
   hang on, hold, honor a bill, impute to, lay to, let on,
   make acknowledgments of, not oppose, offer thanks, open up,
   out with it, own, own up, pay for, pay the bill, pay the piper,
   pin on, pinpoint, place upon, plead guilty, point to, proclaim,
   publish, react, receive, recognize, redeem, reecho, refer to,
   rejoin, render credit, render thanks, reply, respond, retort,
   return, return answer, return for answer, return thanks, reveal,
   reverberate, riposte, saddle on, saddle with, say, say in reply,
   set down to, settle upon, shoot back, spill, spill it, spit it out,
   stand the costs, swear, talk back, tell, tell all, tell the truth,
   testify, thank, view, vouch, warrant, witness, yield



The Devil's Dictionary (1881-1906):

ACKNOWLEDGE, v.t.  To confess.  Acknowledgement of one another's
faults is the highest duty imposed by our love of truth.