What is Doubt?

Legal Definition
Doubt characterises a status in which the mind remains suspended between two contradictory propositions and unable to assent to either of them. Doubt on an emotional level is indecision between belief and disbelief. Doubt involves uncertainty, distrust or lack of sureness of an alleged fact, an action, a motive, or a decision. Doubt questions a notion of a perceived "reality", and may involve delaying or rejecting relevant action out of concerns for mistakes or faults or appropriateness. (Compare paradox).
-- Wikipedia
Legal Definition
The uncertainty which exists in relation to a fact, a proposition, or other thing; or it is an equipoise of the mind arising from an equality of contrary reasons. Ayl. Pand. 121.

2. The embarrassing position of a judge is that of being in doubt, and it is frequently the lot of the wisest and most enlightened to be in this condition, those who have little or no experience usually find no difficulty in deciding the most, problematical questions.

3. Some rules, not always infallible, have been adopted in doubtful cases, in order to arrive at the truth. 1. In civil cases, the doubt ought to operate against him, who having it in his power to prove facts to remove the doubt, has neglected to do so. In cases of fraud when there is a doubt, the presumption of innocence (q. v.) ought to remove it. 2. In criminal cases, whenever a reasonable doubt exists as to the guilt of the accused that doubt ought to operate in his favor. In such cases, particularly, when the liberty, honor or life of an individual is at stake, the evidence to convict ought to be clear, and devoid of all reasonable doubt. See Best on Pres. §195; Wils. on Cir. Ev. 26; Theory of Presumptive Proof, 64; 33 How. St. Tr. 506; Burnett, Cr. Law of Scotl. 522; 1 Greenl. Ev. §1 D'Aguesseau, Oeuvres, vol. xiii. p. 242; Domat, liv. 3, tit. 6.

4. No judge is presumed to have any doubt on a question of law, and he cannot therefore refuse to give a judgment on that account. 9 M. R. 355; Merlin, Repert. h. t.; Ayliffe's Pand. b. 2, t. 17; Dig. lib. 34, t. 5; Code, lib. 6, t. 38. Indeed, in some countries; in China, for example, ignorance of the law in a judge is punishable with blows. Penal Laws of China, B. 2, s. 61.
-- Bouviers Law Dictionary
Legal Definition
Uncertainty of mind; the absence of a settled opinion or conviction; the attitude of mind towards the acceptance of or belief in a proposition, thcory or statement, in which the judgment is not at rest but inclines alternately to either side. Rowe v. Baber, 98 Ala. 422, 8 South. 865; Smith v. Railway Co.., 143 Mo. 33, 44 S. W. 718; West Jersey Traction Co., v. Camden Horse R. Co.., 52 N. J. Eq. 452, 29 Atl. 333.

Reasonable doubt. This is a term often used, probably pretty well understood, but not easily defined. It does not mean a mere possible doubt, because everything relating to human affaire, and depending on moral evidence, is open to some possible or imaginary doubt. It is that state of the case which, after the entire comparison and consideration of all the evidence, leaves the minds of jurors in that condition that they cannot say they feel an abiding conviction to a moral certainty of the truth of the charge. Donnelly v. State, 26 N. J. Law, 601, 615.

A reasonable doubt is deemed to exist, within the rule that the jury should not convict unless satisfied beyond a reasonable doubt, when the evidence is not sufficient to satisfy the judgment of the truth of a proposition with such certainty that a prudent man would feel safe in acting upon it in his own important affairs. Arnold v. State, 23 Ind. 170.

The burden of proof is upon the prosecutor. All the presumptions of law independent of evidence are in favor of innocence; and every person is presumed to be innocent until he is proved guilty. If upon such proof there is reasonable doubt remaining, the accused is entitled to the benefit of it by an acquittal; for it is not sufficient to establish a probability, though a strong one, arising from the doctrine of chances, that the fact charged is more likely to be true than the contrary, but the evidence must establish the truth of the fact to a reasonable and moral certainty,—a certainty that convinces and directs the understanding and satisfies the reason and judgment of those who are bound to act conscientiously upon it. This is proof beyond reasonable doubt; because if the law, which mostly depends upon considerations of a moral nature, should go further than this, and require absolute certainty, it would exclude circumstantial evidence altogether. Per Shaw, C. J., in Com. v. Webster, 5 Cush. (Mass.) 320, 52 Am. Dec. 711. And see further, Tompkins v. Butterfield (O. C.) 25 Fed. 558; State v. Zdanowicz, 69 N. J. Law, 619, 55 Atl. 748; U. S. v. Youtsey (C. Ct) 91 Fed. 868 ; State v. May, 172 Mo. 630, 72 S. W. 918; Com. v. Childs, 2 Pittsb. R. (Pa.) 400; State v. Hennessy, 55 Iowa, 300, 7 N. W. 641; Harris v. State, 155 Ind. 265, 58 N. E. 75; Knight v. State, 74 Miss. 140, 26 South. 860; Carleton v. State, 43 Neb. 373, 61 N. W. 699; State v. Reed, 62 Me. 129; State v. Ching Ling, 16 Or. 419, 18 Pac. 844; Stout v. State, 90 Ind. 1; Bradley v. State, 31 Ind. 505; Alien v. State, 111 Ala. 80, 20 South 494; State v. Rover, 11 Nev. 344; Jones v. State, 120 Ala. 303, 25 South. 204 ; Siberry v. State, 133 Ind. 677, 33 N. E. 681; Purkey v. State, 3 Heisk. (Tenn.) 28; U. S. v. Post (D. C.) 128 Fed. 957; In S. v. Breese (D. C.) 131 Fed. 917.
-- Black's Law Dictionary