The Collaborative International Dictionary of English v.0.48:

Doubt \Doubt\ (dout), v. i. [imp. & p. p. Doubted; p. pr. &
   vb. n. Doubting.] [OE. duten, douten, OF. duter, doter,
   douter, F. douter, fr. L. dubitare; akin to dubius doubtful.
   See Dubious.]
   1. To waver in opinion or judgment; to be in uncertainty as
      to belief respecting anything; to hesitate in belief; to
      be undecided as to the truth of the negative or the
      affirmative proposition; to b e undetermined.
      [1913 Webster]

            Even in matters divine, concerning some things, we
            may lawfully doubt, and suspend our judgment.
                                                  --Hooker.
      [1913 Webster]

            To try your love and make you doubt of mine.
                                                  --Dryden.
      [1913 Webster]

   2. To suspect; to fear; to be apprehensive. [Obs.]

   Syn: To waver; vacillate; fluctuate; hesitate; demur;
        scruple; question.
        [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Doubt \Doubt\, v. t.
   1. To question or hold questionable; to withhold assent to;
      to hesitate to believe, or to be inclined not to believe;
      to withhold confidence from; to distrust; as, I have heard
      the story, but I doubt the truth of it.
      [1913 Webster]

            To admire superior sense, and doubt their own!
                                                  --Pope.
      [1913 Webster]

            I doubt not that however changed, you keep
            So much of what is graceful.          --Tennyson.
      [1913 Webster]

   To doubt not but.

            I do not doubt but I have been to blame. --Dryden.
      [1913 Webster]

            We doubt not now
            But every rub is smoothed on our way. --Shak.
      [1913 Webster]

   Note: That is, we have no doubt to prevent us from believing,
         etc. (or notwithstanding all that may be said to the
         contrary) -- but having a preventive sense, after verbs
         of "doubting" and "denying" that convey a notion of
         hindrance. --E. A. Abbott.
         [1913 Webster]

   2. To suspect; to fear; to be apprehensive of. [Obs.]
      [1913 Webster]

            Edmond [was a] good man and doubted God. --R. of
                                                  Gloucester.
      [1913 Webster]

            I doubt some foul play.               --Shak.
      [1913 Webster]

            That I of doubted danger had no fear. --Spenser.
      [1913 Webster]

   3. To fill with fear; to affright. [Obs.]
      [1913 Webster]

            The virtues of the valiant Caratach
            More doubt me than all Britain.       --Beau. & Fl.
      [1913 Webster]

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.
      [1913 Webster]

            Doubt is the beginning and the end of our efforts to
            know.                                 --Sir W.
                                                  Hamilton.
      [1913 Webster]

            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.
      [1913 Webster]

   2. Uncertainty of condition.
      [1913 Webster]

            Thy life shall hang in doubt before thee. --Deut.
                                                  xxviii. 66.
      [1913 Webster]

   3. Suspicion; fear; apprehension; dread. [Obs.]
      [1913 Webster]

            I stand in doubt of you.              --Gal. iv. 20.
      [1913 Webster]

            Nor slack her threatful hand for danger's doubt.
                                                  --Spenser.
      [1913 Webster]

   4. Difficulty expressed or urged for solution; point
      unsettled; objection.
      [1913 Webster]

            To every doubt your answer is the same. --Blackmore.
      [1913 Webster]

   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):

doubt
    n 1: the state of being unsure of something [syn: doubt,
         uncertainty, incertitude, dubiety, doubtfulness,
         dubiousness] [ant: certainty]
    2: uncertainty about the truth or factuality or existence of
       something; "the dubiousness of his claim"; "there is no
       question about the validity of the enterprise" [syn: doubt,
       dubiousness, doubtfulness, question]
    v 1: consider unlikely or have doubts about; "I doubt that she
         will accept his proposal of marriage"
    2: lack confidence in or have doubts about; "I doubt these
       reports"; "I suspect her true motives"; "she distrusts her
       stepmother"