Webster's Dictionary 
(1): ( a.) Capable of being proved.
(2): ( a.) Having more evidence for than against; supported by evidence which inclines the mind to believe, but leaves some room for doubt; likely.
(3): ( a.) Rendering probable; supporting, or giving ground for, belief, but not demonstrating; as, probable evidence; probable presumption.
Cyclopedia of Biblical, Theological and Ecclesiastical Literature 
(Lat. probabilis), a barbarous technical word which serves to designate the philosophic dogma that anything which does not admit of demonstration may admit the probable as proof, if such a course does not involve absurdity or contradiction. "As demonstration," says Locke, "is the showing the agreement or disagreement of two ideas, by the intervention of one or more proofs, which have a constant, immutable, and visible connection one with another; so probability is nothing but the appearance of such an agreement or disagreement by the intervention of proofs whose connection is not constant and immutable, or at least is not perceived to be so, but is, or appears for the most part to be so, and is enough to induce the mind to judge the proposition to be true or false, rather than the contrary ... The entertainment the mind gives this sort of propositions is called belief, assent, or opinion, which is admitting or receiving any proposition as true upon arguments or proofs that are found to persuade us to receive it as true, without certain knowledge that it is so. And herein lies the difference between probability and certainty, faith and knowledge, that in all the parts of knowledge there is intuition; each immediate idea, each step, has its visible and certain connection; in belief not so. That which makes us believe is something extraneous to the thing we believe; something not evidently joined on both sides to, and so not manifestly showing the agreement or disagreement of, those ideas that are under consideration" (Essay on the Human Understanding, bk. 4, ch. 15; comp. Reid, Intel. Powers, essay 7, ch. 3). "The word probable," — says Mr. Stewart, "does not imply any deficiency in the proof, but only marks the particular nature of that proof, as contradistinguished from another species of evidence. It is opposed not to what is certain, but to what admits of being demonstrated after the manner of the mathematicians.
This differs widely from the meaning annexed to the same word in popular discourse; according to which, whatever event is said to be probable is understood to be expected with some degree of doubt... But although, in philosophical language, the epithet probable be applied to events which are acknowledged to be certain, it is also applied to events which are called probable by the vulgar. The philosophical meaning of the word, therefore, is more comprehensive than the popular; the former denoting that particular species of evidence of which contingent truths admit; the latter being confined to such degrees of this evidence as fall short of the highest. These different degrees of probability the philosopher considers as a series, beginning with bare possibility, and terminating in that apprehended infallibility with which the phrase moral certainty is synonymous. To this last term of the series the word probable is, in its ordinary acceptation, plainly inapplicable" (Elements, pt. 2, ch. 2, § 4).
Archbishop Butler, in his treatment of the evidences of Christianity, has had frequent recourse to this theory of the probable, and in consequence has at times laid himself open to severe attacks from the deistical and infidel schools of philosophy. By dwelling exclusively upon the absence of direct contradiction, and sinking the absence of confirmation, the learned author of the Analogy not unfrequently converts absolute ignorance into the likeness of some degree of positive knowledge. So Campbell, who borrowed from Butler, constructed most ingenious arguments on this paradox. Both these English thinkers seem to have had a confused notion that the improbability is an actual thing which still exists. Thus Campbell, after Butler, says, e.g., "The chances that a comet will not appear at a given instant in a given place are infinite. The presumption against the statement is therefore as strong as experience can afford; and yet when an astronomer announces the appearance of the comet you unhesitatingly believe him." The object in this statement is to prove that we must depend largely upon testimony built up from experience, and that therefore knowledge is built upon the parobable. The result is, of course, a delusive appearance of independent scientific grounds for what is really a purely a prioni deduction. Like methods are now adopted in scientific circles, and what Hume and consorts once condemned the theologians for, the latter now have to contend with in the application of scientific queryings to the positive in divine laws, and institutions. See Ueberweg, Hist. of Philos. (Index in vol. 2); The (Lond.) Quatr. Rev. Jan. 1875, p. 31 sq.; London Academy, Nov. 15, 1873, p. 435, Colossians 1; Stephen, Religious Thought in England in the 18th Century, vol. 1.