noun (n)

• a defense of some offensive behavior or some failure to keep a promise etc.(noun.communication)

  Synonym:
  alibi, excuse, self-justification
  

  source: wordnet30
• the act of freeing from guilt or blame(noun.act)

  source: wordnet30
• The act of exculpating from alleged fault or crime; that which exculpates; excuse.(noun)

  source: webster1913