Definition - What does Affirmative Defense mean?
An affirmative defense is a defense which can relieve the defendant from liability even if the accusations made are true. Affirmative defenses require that new evidence be introduced. This new evidence should justify the defendant's actions in a legal sense to remove culpability. Examples of affirmative defenses include self-defense, entrapment, necessity, and insanity.
Justipedia explains Affirmative Defense
Affirmative defenses serve to remove legal responsibility for defendants when there is a legitimate and legal reason for their actions. For example, if a man is being sued for assaulting another man, and if the man being sued can prove that he was acting in self defense because the man making the accusations was trying to rob him, this would be an affirmative defense. The defendant is not trying to deny that the accusation of assault is true. He is simply trying to show that there was a good justification for his actions.