An alibi is a defense arguing that the defendant was not present at the scene of the crime when it occurred, and therefor cannot be guilty of it. The term is derived from a Latin word, which means literally "somewhere else."
While most criminal defenses intend to excuse, justify or exculpate the defendant's actions, they do not dispute that the defendant was present at the scene of the crime and committed the actions for which he is being tried. These defenses, such as consent, duress, mistake of fact and so on, attempt to lessen the defendant's sentence or prove her innocence by addressing the mens rea element of the crime. The alibi defense is unique from these, in that it attempts to eliminate actus reus altogether by arguing that the defendant could not possibly have committed the crime, by virtue of having been elsewhere. Alibis are proven in court by the defense's witnesses, who testify that the defendant was truly elsewhere at the time of the crime, and sometimes by other evidence such as security camera recordings.
For example, if George is accused of attacking and injuring a defenseless old woman in an alleyway on the night of June first, his defense may present the alibi that he could not have committed the crime because he was in fact at the local pub. The defense will call witnesses to the stand who were also at the pub on the night of June first and these witnesses will, George hopes, testify that he was in fact drinking with them at the time the crime was committed. After several of these testimonies, the alibi will probably be sustained, the charges will probably be dropped and George will be free to go.