The Collaborative International Dictionary of English v.0.48:

Exculpation \Ex`cul*pa"tion\, n. [Cf. LL. exculpatio.]
   The act of exculpating from alleged fault or crime; that
   which exculpates; excuse.
   [1913 Webster]

         These robbers, however, were men who might have made
         out a strong case in exculpation of themselves.
                                                  --Southey.
   [1913 Webster]

WordNet (r) 3.0 (2006):

exculpation
    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: the act of freeing from guilt or blame

Moby Thesaurus II by Grady Ward, 1.0:

39 Moby Thesaurus words for "exculpation":
   absolution, acquittal, acquittance, amnesty, clearance, clearing,
   compurgation, destigmatization, destigmatizing, discharge,
   disculpation, dismissal, excuse, exemption, exoneration,
   explanation, forgiveness, grace, immunity, indemnity,
   justification, pardon, purgation, purging, quietus, quittance,
   rationalization, redemption, rehabilitation, reinstatement,
   release, remission, remission of sin, reprieve, restoration,
   shrift, sparing, verdict of acquittal, vindication