What is Alibi?

Legal Definition
An alibi is a form of defense used in criminal procedure wherein the accused attempts to prove that he or she was in some other place at the time the alleged offense was committed. The Criminal Law Deskbook of Criminal Procedure states: "Alibi is different from all of the other defenses; it is based upon the premise that the defendant is truly innocent." In the Latin language alibī means "somewhere else."
-- Wikipedia
Legal Definition
N. A defense to a criminal charge alleging that the accused was somewhere other than at the scene of the crime at the time it occurred.

v. To provide an alibi for someone.
Illustrative caselaw
See, e.g. Lee v. Kemna, 534 U.S. 362 (2002).
See also
Legal Definition
In evidence. This is a Latin word which signifies, elsewhere.

2. When a person, charged with a crime, proves (se eadem die fuisse alibi,) that he was, at the time alleged, in a different place from that in which it was committed, he is said to prove an alibi, the effect of which is to lay a founation for the necessary inference, that he could not have committed it. See Bract. fo. 140, lib. 3, cap. 20, De Corona.

3. This proof is usually made out by the testimony of witnesses, but it is presumed it might be made out by writings; as if the party could prove by a record properly authenticated, that on the day or at the time in question, he was in another place.

4. It must be admitted that mere alibi evidence lies under a great and general prejudice, and ought to be heard with un-common caution; but if it appear, to be founded in truth, it is the best negative evidence that can be offered; it is really positive evidence, which in the nature of things necessarily implies a negative; and in many cases it is the only evidence which an innocent man can offer.
-- Bouviers Law Dictionary
Legal Definition
Lat. In criminal law. Elsewhere; in another place. A term used to express that mode of defense to a criminal prosecution, where the party accused, in order to prove that he could not have committed the crime with which he is charged, offers evidence to show that he was in another place at the time; which is termed setting up an alibi. State v. McGarry, 111 Iowa, 709, 83 N. W. 718; State v. Child, 40 Kan. 482, 20 Pac. 275; State v. Powers, 72 Vt. 168, 47 Atl. 830; Peyton v. State, 54 Neb. 188, 74 N. W. 597.
-- Black's Law Dictionary
Legal Definition
At a place other than one designated.
-- Ballentine's Law Dictionary