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.
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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]