a. [ L. improbabilis; pref. im- not + probabilis probable: cf. F. improbable. See Probable. ] Not probable; unlikely to be true; not to be expected under the circumstances or in the usual course of events; as, an improbable story or event. [ 1913 Webster ]

He . . . sent to Elutherius, then bishop of Rome, an improbable letter, as some of the contents discover. Milton.

-- Im*prob"a*ble*ness, n. -- Im*prob"a*bly, adv. [ 1913 Webster ]

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″ is the probable error in a given case, the chances that the real error is greater than 3″ 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 ]