The Collaborative International Dictionary of English v.0.48:

Doubt \Doubt\, n. [OE. dute, doute, F. doute, fr. douter to
   doubt. See Doubt, v. i.]
   1. A fluctuation of mind arising from defect of knowledge or
      evidence; uncertainty of judgment or mind; unsettled state
      of opinion concerning the reality of an event, or the
      truth of an assertion, etc.; hesitation.
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            Doubt is the beginning and the end of our efforts to
            know.                                 --Sir W.
                                                  Hamilton.
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            Doubt, in order to be operative in requiring an
            acquittal, is not the want of perfect certainty
            (which can never exist in any question of fact) but
            a defect of proof preventing a reasonable assurance
            of quilt.                             --Wharton.
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   2. Uncertainty of condition.
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            Thy life shall hang in doubt before thee. --Deut.
                                                  xxviii. 66.
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   3. Suspicion; fear; apprehension; dread. [Obs.]
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            I stand in doubt of you.              --Gal. iv. 20.
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            Nor slack her threatful hand for danger's doubt.
                                                  --Spenser.
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   4. Difficulty expressed or urged for solution; point
      unsettled; objection.
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            To every doubt your answer is the same. --Blackmore.
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   No doubt, undoubtedly; without doubt.

   Out of doubt, beyond doubt. [Obs.] --Spenser.

   Syn: Uncertainty; hesitation; suspense; indecision;
        irresolution; distrust; suspicion; scruple; perplexity;
        ambiguity; skepticism.
        [1913 Webster]

WordNet (r) 3.0 (2006):

no doubt
    adv 1: admittedly; "to be sure, he is no Einstein" [syn: to be
           sure, without doubt, no doubt]

Moby Thesaurus II by Grady Ward, 1.0:

28 Moby Thesaurus words for "no doubt":
   admittedly, beyond doubt, beyond question, by all odds,
   dollars to doughnuts, doubtless, doubtlessly, in all likelihood,
   in all probability, incontestably, indisputably, indubitably,
   irrefragably, irrefutably, like enough, likely, most likely,
   out of question, presumably, presumptively, probably, ten to one,
   undeniably, undoubtedly, unquestionably, very like, very likely,
   without doubt