Excuse, Justification, Exculpation
Exculpation is the act of being cleared of blame, and excuse and justification are the most common criminal defenses that achieve this. In the the criminal law system of the United States, excuse and justification are most commonly employed in affirmative defenses that provide rationale for finding the defendant not guilty, even though he committed an actus reus, possessed the necessary state of mind, and caused the damage to society that would normally constitute a criminal offense. The exculpation of guilt - in situations where it is justified or excusable - is considered to be more desirable to society than the rote prosecution of specific crimes.
Justifications are defenses that focus primarily on the criminal offense that was committed by the defendant. A criminal offense may be justified if it in some way benefits society or upholds principles that society values highly. For example, assault and battery could be fully justifiable if those actions are shown to be in self defense. It is a fundamental value of society that everyone has a right to defend themselves if they come under attack, and so this behavior is in many situations justified.
Excuse defenses are oriented toward the actor, rather than the criminal act. Even though the actions of the defendant were criminal, intentional and caused some level of harm to society, these defenses may exculpate criminal guilt because for some extenuating reason the defendant is found not to be responsible for his own actions. If the defendant was involuntarily drugged and, while intoxicated, injures another person, her behavior may be excused and exculpated.
The reach of justification and excuse only goes so far. Some crimes are so serious that full exculpation is not possible, though excuse and justification may lessen criminal guilt so some degree. The duress defense, for example, may excuse criminal actions committed by the defendant if he was being threatened by another person and reasonably feared for his life: theft under these circumstances might be fully excusable, but taking the life of an innocent person is almost certainly not. In this case, the duress defense could probably not exculpate the defendant fully of criminal guilt, but might serve to reduce liability from murder to manslaughter.