Source : Webster's Revised Unabridged Dictionary (1913)

Alibi \Al"i*bi\, n. [L., elsewhere, at another place. See
   {Alias}.] (Law)
   The plea or mode of defense under which a person on trial for
   a crime proves or attempts to prove that he was in another
   place when the alleged act was committed; as, to set up an
   alibi; to prove an alibi.

Source : WordNet®

alibi
     n 1: (law) a defense by an accused person purporting to show that
          he or she could not have committed the crime in question
     2: a defense of some offensive behavior or some failure to keep
        a promise etc.; "he kept finding excuses to stay"; "every
        day he had a new alibi for not getting a job"; "his
        transparent self-justification was unacceptable" [syn: {excuse},
         {exculpation}, {self-justification}]
     v : exonerate by means of an alibi