The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

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, alibi,
       exculpation, self-justification]
    v 1: exonerate by means of an alibi

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "alibi":
   alibi out of, apologize for, apology, blind, cloak, color, cop-out,
   cover, cover story, cover with excuses, cover-up, device, excuse,
   explain, explanation, facade, feint, front, gloss, guise, handle,
   lame excuse, lie out of, likely story, locus standi,
   make apology for, mask, offer excuse for, ostensible motive, out,
   plea, plead ignorance, poor excuse, pretense, pretension, pretext,
   protestation, public motive, put-off, refuge, right, screen,
   semblance, sham, show, smoke screen, squirm out of, stalking-horse,
   stratagem, subterfuge, trick, varnish, veil, worm out of



Bouvier's Law Dictionary, Revised 6th Ed (1856):

ALIBI, 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 foundation 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 uncommon 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.