The Collaborative International Dictionary of English v.0.48:

Probable \Prob"a*ble\, a. [L. probabilis, fr. probare to try,
   approve, prove: cf. F. probable. See Prove, and cf.
   Provable.]
   [1913 Webster]
   1. Capable of being proved. [Obs.]
      [1913 Webster]

   2. Having more evidence for than against; supported by
      evidence which inclines the mind to believe, but leaves
      some room for doubt; likely.
      [1913 Webster]

            That is accounted probable which has better
            arguments producible for it than can be brought
            against it.                           --South.
      [1913 Webster]

            I do not say that the principles of religion are
            merely probable; I have before asserted them to be
            morally certain.                      --Bp. Wilkins.
      [1913 Webster]

   3. Rendering probable; supporting, or giving ground for,
      belief, but not demonstrating; as, probable evidence;
      probable presumption. --Blackstone.
      [1913 Webster]

   Probable cause (Law), a reasonable ground of presumption
      that a charge is, or my be, well founded.

   Probable error (of an observation, or of the mean of a
      number), that within which, taken positively and
      negatively, there is an even chance that the real error
      shall lie. Thus, if 3[sec] is the probable error in a
      given case, the chances that the real error is greater
      than 3[sec] are equal to the chances that it is less. The
      probable error is computed from the observations made, and
      is used to express their degree of accuracy.

   The probable, that which is within the bounds of
      probability; that which is not unnatural or preternatural;
      -- opposed to the marvelous.
      [1913 Webster]

WordNet (r) 3.0 (2006):

probable
    adj 1: likely but not certain to be or become true or real; "a
           likely result"; "he foresaw a probable loss" [syn:
           probable, likely] [ant: improbable, unlikely]
    2: apparently destined; "the probable consequences of going
       ahead with the scheme"
    n 1: an applicant likely to be chosen

Moby Thesaurus II by Grady Ward, 1.0:

90 Moby Thesaurus words for "probable":
   anticipatable, anticipated, apparent, approaching, apt, awaited,
   believable, calculable, cogitable, colorable, coming, conceivable,
   conceivably possible, contingent, credible, desired, destinal,
   destined, determined, divinable, due, earthly, emergent, eventual,
   evident, expected, extrapolated, fair, fatal, fated, fatidic,
   feasible, foreknowable, foreseeable, foreseen, foretellable,
   forthcoming, future, futuristic, hereafter, hoped-for, hopeful,
   humanly possible, illusory, imaginable, imminent, improbable,
   in prospect, in the cards, in view, indubitable, later, liable,
   likely, long-expected, mortal, most likely, nearing, odds-on,
   on the horizon, ostensible, overdue, planned, plausible, plotted,
   possible, potential, precognizable, predictable,
   predictable within limits, predicted, presumable, presumed,
   presumptive, projected, promised, promising, prophesied,
   prospective, rational, reasonable, seeming, statistically probable,
   thinkable, to come, to-be, ultimate, undoubted, unquestionable,
   verisimilar



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

PROBABLE. That which has the appearance of truth; that which appears to be
founded in reason.