The Collaborative International Dictionary of English v.0.48:

Probability \Prob`a*bil"i*ty\, n.; pl. Probabilities. [L.
   probabilitas: cf. F. probabilit['e].]
   [1913 Webster]
   1. The quality or state of being probable; appearance of
      reality or truth; reasonable ground of presumption;
      likelihood.
      [1913 Webster]

            Probability is the appearance of the agreement or
            disagreement of two ideas, by the intervention of
            proofs whose connection is not constant, but appears
            for the most part to be so.           --Locke.
      [1913 Webster]

   2. That which is or appears probable; anything that has the
      appearance of reality or truth.
      [1913 Webster]

            The whole life of man is a perpetual comparison of
            evidence and balancing of probabilities.
                                                  --Buckminster.
      [1913 Webster]

            We do not call for evidence till antecedent
            probabilities fail.                   --J. H.
                                                  Newman.
      [1913 Webster]

   3. (Math.) Likelihood of the occurrence of any event in the
      doctrine of chances, or the ratio of the number of
      favorable chances to the whole number of chances,
      favorable and unfavorable. See 1st Chance, n., 5.
      [1913 Webster]

   Syn: Likeliness; credibleness; likelihood; chance.
        [1913 Webster]

WordNet (r) 3.0 (2006):

probability
    n 1: a measure of how likely it is that some event will occur; a
         number expressing the ratio of favorable cases to the whole
         number of cases possible; "the probability that an unbiased
         coin will fall with the head up is 0.5" [syn:
         probability, chance]
    2: the quality of being probable; a probable event or the most
       probable event; "for a while mutiny seemed a probability";
       "going by past experience there was a high probability that
       the visitors were lost" [ant: improbability,
       improbableness]

Moby Thesaurus II by Grady Ward, 1.0:

187 Moby Thesaurus words for "probability":
   a thing for, accidentality, actuarial calculation,
   actuarial prediction, adventitiousness, affinity, aftertime,
   afteryears, anticipation, apocalypse, aptitude, aptness,
   bare possibility, bent, best bet, bias, break, by-and-by, cast,
   casualness, certainty, chance, chances, conatus, conceivability,
   conceivableness, conduciveness, confidence, contemplation,
   contingency, course ahead, crystal ball, delight, destiny,
   determinism, diathesis, disposition, distant future, eagerness,
   even chance, eventuality, expectancy, expectation, fate,
   favorable prospect, feeling for, flukiness, foreboding, forecast,
   forecasting, foreshowing, foresight, foretelling, fortuitousness,
   fortuity, fortune, forward look, future, future tense, futurism,
   futurity, gamble, good chance, good fortune, good luck,
   good opportunity, good possibility, guesswork, hap, happenstance,
   happy chance, heedless hap, hereafter, hope, how they fall,
   immediate future, immediate prospect, imminence, improbability,
   inclination, indeterminacy, indeterminateness, law of averages,
   leaning, liability, liableness, likelihood, likeliness, liking,
   lot, luck, main chance, moira, near future, obligation, odds,
   odds-on, odds-on chance, off chance, offing, omen, opportunity,
   outlook, outside chance, outside hope, penchant, possibility,
   possibleness, posteriority, potential, potentiality, prediction,
   predilection, predisposition, prefiguration, prefigurement,
   prefiguring, prejudice, presage, presaging, presentiment,
   preshowing, presignifying, presumption, principle of indeterminacy,
   problematicness, proclivity, prognosis, prognostication, project,
   promise, proneness, propensity, prophecy, prophesying, prospect,
   prospectus, random sample, readiness, reliance, remote possibility,
   risk, run of luck, sensitivity to, serendipity, small hope,
   soft spot, soothsay, speculation, sporting chance,
   statistical prediction, statistical probability, sure bet,
   sure thing, susceptibility, tendency, the attainable, the breaks,
   the feasible, the future, the morrow, the possible,
   the sweet by-and-by, theory of probability, thinkability,
   thinkableness, thought, time ahead, time just ahead, time to come,
   tomorrow, tropism, turn, twist, unastonishment, uncertainty,
   uncertainty principle, vaticination, virtuality, warp, weakness,
   well-grounded hope, what is possible, what may be, what might be,
   whatever comes, willingness



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

PROBABILITY. That which is likely to happen; that which is most consonant to
reason; for example, there is a strong probability that a man of a good
moral character, and who has heretofore been remarkable for truth, will,
when examined as a witness under oath, tell the truth; and, on the contrary,
that a man who has been guilty of perjury, will not, under the same
circumstances, tell the truth; the former will, therefore, be entitled to
credit, while the latter will not.