The Collaborative International Dictionary of English v.0.48:

Uncertainty \Un*cer"tain*ty\, n.; pl. Uncertainties.
   [1913 Webster]
   1. The quality or state of being uncertain.
      [1913 Webster]

   2. That which is uncertain; something unknown.
      [1913 Webster]

            Our shepherd's case is every man's case that quits a
            moral certainty for an uncertainty.   --L'Estrange.
      [1913 Webster]

WordNet (r) 3.0 (2006):

uncertainty
    n 1: being unsettled or in doubt or dependent on chance; "the
         uncertainty of the outcome"; "the precariousness of his
         income" [syn: uncertainty, uncertainness,
         precariousness] [ant: certainty, foregone conclusion,
         sure thing]
    2: the state of being unsure of something [syn: doubt,
       uncertainty, incertitude, dubiety, doubtfulness,
       dubiousness] [ant: certainty]

Moby Thesaurus II by Grady Ward, 1.0:

231 Moby Thesaurus words for "uncertainty":
   Pyrrhonism, accidentality, actuarial calculation, adventitiousness,
   agitation, ambiguity, ambiguousness, ambitendency, ambivalence,
   amphibology, anxiety, apprehension, arrhythmia, bleariness, blur,
   blurriness, bother, break, brokenness, calculated risk,
   capriciousness, casualness, chance, chanciness, change of mind,
   changeableness, choppiness, cliff-hanging, complexity of meaning,
   concern, criticalness, darkness, defocus, delicacy, destiny,
   desultoriness, deviability, diffidence, dimness, disconnectedness,
   discontinuity, disquiet, distress, distrust, distrustfulness,
   double entendre, double meaning, double reference,
   double-mindedness, doubt, doubtfulness, dread, dubiety,
   dubiousness, eccentricity, equivocacy, equivocality, equivocalness,
   erraticism, erraticness, expectant waiting, faintness, fate,
   feebleness, fence-sitting, fence-straddling, fibrillation,
   fickleness, filminess, fitfulness, fits and starts, flier,
   flightiness, fluctuation, flukiness, fogginess, fortuitousness,
   fortuity, fortune, freakishness, fuzziness, gamble, good fortune,
   good luck, half-belief, half-visibility, hap, happenstance,
   happy chance, hazard, hazardousness, haziness, heedless hap,
   hesitation, how they fall, impulsiveness, incertitude,
   inconsistency, inconstancy, indecision, indecisiveness,
   indefiniteness, indeterminacy, indeterminateness, indistinctness,
   indistinguishability, infirmity of purpose, insecurity,
   instability, intermittence, irony, irregularity, irresolution,
   jerkiness, law of averages, leeriness, levels of meaning, lot,
   low profile, luck, mercuriality, misdoubt, misgiving, mistiness,
   mistrust, mistrustfulness, moira, moodiness, mugwumpery,
   mugwumpism, multivocality, opportunity, paleness, paronomasia,
   patchiness, perilousness, perturbation, pessimism, play, plunge,
   polysemousness, polysemy, precariousness,
   principle of indeterminacy, probability, problematicness, punning,
   query, question, random sample, reserve, restlessness,
   richness of meaning, risk, riskiness, roughness, run of luck, salt,
   scruple, scrupulousness, second thoughts, self-doubt,
   semivisibility, serendipity, shadow of doubt, shadowiness,
   shakiness, shiftiness, skepticalness, skepticism, slipperiness,
   soft focus, spasticity, speculation, sporadicity, sporadicness,
   spottiness, stagger, statistical probability, suspense, suspicion,
   suspiciousness, sword of Damocles, tergiversation, the breaks,
   theory of probability, ticklish business, ticklishness,
   total skepticism, totteriness, touchiness, trouble,
   uncertainty principle, unclearness, undecidedness, undependability,
   undeterminedness, uneasiness, unevenness, unfixedness,
   unhealthiness, unmethodicalness, unplainness, unpredictability,
   unreliability, unsafeness, unsettledness, unsettlement,
   unsoundness, unstableness, unsteadfastness, unsteadiness,
   unsureness, unsystematicness, untrustworthiness, vague appearance,
   vagueness, variability, variation, variety, venture, waiting,
   wantonness, wariness, waywardness, weakness, whatever comes,
   whimsicality, wobble, wonder, worry



Bouvier's Law Dictionary, Revised 6th Ed (1856):

UNCERTAINTY. That which is unknown or vague. Vide Certainty.