The Collaborative International Dictionary of English v.0.48:

Excuse \Ex*cuse"\, v. t. [imp. & p. p. Excused; p. pr. & vb.
   n. Excusing.] [OE. escusen, cusen, OF. escuser, excuser, F.
   excuser, fr. L. excusare; ex out + causa cause, causari to
   plead. See Cause.]
   1. To free from accusation, or the imputation of fault or
      blame; to clear from guilt; to release from a charge; to
      justify by extenuating a fault; to exculpate; to absolve;
      to acquit.
      [1913 Webster]

            A man's persuasion that a thing is duty, will not
            excuse him from guilt in practicing it, if really
            and indeed it be against Gog's law.   --Abp. Sharp.
      [1913 Webster]

   2. To pardon, as a fault; to forgive entirely, or to admit to
      be little censurable, and to overlook; as, we excuse
      irregular conduct, when extraordinary circumstances appear
      to justify it.
      [1913 Webster]

            I must excuse what can not be amended. --Shak.
      [1913 Webster]

   3. To regard with indulgence; to view leniently or to
      overlook; to pardon.
      [1913 Webster]

            And in our own (excuse some courtly stains.)
            No whiter page than Addison remains.  --Pope.
      [1913 Webster]

   4. To free from an impending obligation or duty; hence, to
      disengage; to dispense with; to release by favor; also, to
      remit by favor; not to exact; as, to excuse a forfeiture.
      [1913 Webster]

            I pray thee have me excused.          --xiv. 19.
      [1913 Webster]

   5. To relieve of an imputation by apology or defense; to make
      apology for as not seriously evil; to ask pardon or
      indulgence for.
      [1913 Webster]

            Think ye that we excuse ourselves to you? --2 Cor.
                                                  xii. 19.

   Syn: To vindicate; exculpate; absolve; acquit.

   Usage: - To Pardon, Excuse, Forgive. A superior pardons
          as an act of mercy or generosity; either a superior or
          an equal excuses. A crime, great fault, or a grave
          offence, as one against law or morals, may be
          pardoned; a small fault, such as a failure in social
          or conventional obligations, slight omissions or
          neglects may be excused. Forgive relates to offenses
          against one's self, and punishment foregone; as, to
          forgive injuries or one who has injured us; to pardon
          grave offenses, crimes, and criminals; to excuse an
          act of forgetfulness, an unintentional offense. Pardon
          is also a word of courtesy employed in the sense of
          excuse.
          [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Excuse \Ex*cuse"\, n. [Cf. F. excuse. See Excuse, v. t.]
   1. The act of excusing, apologizing, exculpating, pardoning,
      releasing, and the like; acquittal; release; absolution;
      justification; extenuation.
      [1913 Webster]

            Pleading so wisely in excuse of it.   --Shak.
      [1913 Webster]

   2. That which is offered as a reason for being excused; a
      plea offered in extenuation of a fault or irregular
      deportment; apology; as, an excuse for neglect of duty;
      excuses for delay of payment.
      [1913 Webster]

            Hence with denial vain and coy excuse. --Milton.
      [1913 Webster]

   3. That which excuses; that which extenuates or justifies a
      fault. "It hath the excuse of youth." --Shak.
      [1913 Webster]

            If eyes were made for seeing.
            Then beauty is its own excuse for being. --Emerson.

   Syn: See Apology.
        [1913 Webster]

WordNet (r) 3.0 (2006):

excuse
    n 1: 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]
    2: a note explaining an absence; "he had to get his mother to
       write an excuse for him"
    3: a poor example; "it was an apology for a meal"; "a poor
       excuse for an automobile" [syn: apology, excuse]
    v 1: accept an excuse for; "Please excuse my dirty hands" [syn:
         excuse, pardon]
    2: grant exemption or release to; "Please excuse me from this
       class" [syn: excuse, relieve, let off, exempt]
    3: serve as a reason or cause or justification of; "Your need to
       sleep late does not excuse your late arrival at work"; "Her
       recent divorce may explain her reluctance to date again"
       [syn: excuse, explain]
    4: defend, explain, clear away, or make excuses for by
       reasoning; "rationalize the child's seemingly crazy
       behavior"; "he rationalized his lack of success" [syn:
       apologize, apologise, excuse, justify, rationalize,
       rationalise]
    5: ask for permission to be released from an engagement [syn:
       excuse, beg off]
    6: excuse, overlook, or make allowances for; be lenient with;
       "excuse someone's behavior"; "She condoned her husband's
       occasional infidelities" [syn: excuse, condone]

Moby Thesaurus II by Grady Ward, 1.0:

187 Moby Thesaurus words for "excuse":
   abject apology, absolution, absolve, acknowledgment, acquit,
   acquittal, acquittance, alibi, alibi out of, allow, amnesty,
   apologize for, apology, basis, be blind to, blind, breast-beating,
   cause, clear, clearance, clearing, cloak, color, compurgation,
   condonation, condone, confession, contrition, cop-out, cover,
   cover story, cover with excuses, cover-up, decontaminate, defence,
   defend, defense, destigmatization, destigmatize, destigmatizing,
   device, discharge, disculpation, dismiss, dismissal, dispense,
   dispense from, dispense with, disregard, escape, evasion, except,
   exculpate, exculpation, exempt, exempt from, exemption, exonerate,
   exoneration, explain, explanation, extenuate, extenuation, facade,
   feint, forgive, forgiveness, foundation, free, front,
   give absolution, give dispensation from, gloss, gloss over, grace,
   grant amnesty to, grant forgiveness, grant immunity,
   grant remission, grounds, guise, handle, heedlessness, ignore,
   immunity, indemnity, indulgence, justification, justify,
   lame excuse, let go, let off, liberate, lie out of, likely story,
   locus standi, loophole, make apology for, makeshift, mask,
   mea culpa, mitigate, mitigation, nonpros, offer excuse for,
   ostensible motive, out, overlook, palliate, palliation, pardon,
   pass over, penitence, permit, plea, plead ignorance, poor excuse,
   pretense, pretension, pretext, protestation, public motive,
   purgation, purge, purging, put-off, quash the charge, quietus,
   quittance, rational ground, rationale, rationalization,
   rationalize, reason, reason for, reason why, redemption, refuge,
   regrets, release, relieve, remise, remission, remission of sin,
   remit, reprieve, right, save the necessity, screen, semblance,
   set free, sham, shift, show, shrift, shrive, shrug off,
   smoke screen, spare, sparing, squirm out of, stalking-horse, stall,
   stated cause, stopgap, story, stratagem, substitute, subterfuge,
   the big idea, the idea, the whatfor, the wherefore, the why, trick,
   underlying reason, varnish, veil, verdict of acquittal, vindicate,
   vindication, warrant, way out, whitewash, wink at,
   withdraw the charge, worm out of



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

EXCUSE. A reason alleged for the doing or not doing a thing. This word
presents two ideas differing essentially from each other. In one case an
excuse may be made in, order to own that the party accused is not guilty; in
another, by showing that though guilty, he is less so, than he appears to
be. Take, for example, the case of a sheriff who has an execution against an
individual, and who in performance of his duty, arrests him; in an action by
the defendant against the sheriff, the latter may prove the facts, and this
shall be a sufficient excuse for him: this is an excuse of the first kind,
or a complete justification; the sheriff was guilty of no offence. But
suppose, secondly, that the sheriff has an execution against Paul, and by
mistake, and without any malicious design, be arrests Peter instead of Paul;
the fact of his having the execution against Paul and the mistake being
made, will not justify the sheriff, but it will extenuate and excuse his
conduct, and this will be an excuse of the second kind.
     3. Persons are sometimes excused for the commission of acts, which
ordinarily are crimes, either because they had no intention of doing wrong,
or because they had no power of judging, and therefore had no criminal will
(q.v.); or having power, of judging they had no choice, and were compelled
by necessity. Among the first class may be placed infants under the age of
discretion, lunatics, and married women committing an offence in the
presence of their husbands, not malum in se, as treason or murder; 1 Hale's
P. C. 44, 45 or in offences relating to the domestic concern or management
of the house, as the keeping of a bawdy house. Hawk. b. 1, c. 1, s. 12.
Among acts of the second kind may be classed, the beating or killing another
in self-defence; the destruction of property in order to prevent a more
serious calamity, as the tearing down of a house on fire, to prevent its
spreading to the neighboring property, and the like. See Dalloz, Dict. h.t.