Source : Webster's Revised Unabridged Dictionary (1913)

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.

            Doubt is the beginning and the end of our efforts to
            know.                                 --Sir W.
                                                  Hamilton.

            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.

   2. Uncertainty of condition.

            Thy life shall hang in doubt before thee. --Deut.
                                                  xxviii. 66.

   3. Suspicion; fear; apprehension; dread. [Obs.]

            I stand in doubt of you.              --Gal. iv. 20.

            Nor slack her threatful hand for danger's doubt.
                                                  --Spenser.

   4. Difficulty expressed or urged for solution; point
      unsettled; objection.

            To every doubt your answer is the same. --Blackmore.

   {No doubt}, undoubtedly; without doubt.

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

   Syn: Uncertainty; hesitation; suspense; indecision;
        irresolution; distrust; suspicion; scruple; perplexity;
        ambiguity; skepticism.

Source : WordNet®

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