Alibi is a Latin term meaning "in another place." It is an excuse supplied by a person suspected of or charged with a crime, supposedly explaining why they couldn't be guilty. An alibi may be posited by the accused, or they may produce a third person to supply an alibi for them.
An alibi generally involves a claim that the accused was involved in another activity at the time of the crime. It is a defense that claims the accused is innocent, as opposed to merely excused from blame.