Dictionary.net

An explanation for a defendant's actions that excuses or justifies his behavior. For example, acting in self-defense is a common affirmative defense to a charge of battery or homicide. Other affirmative defenses include insanity, duress and intoxication.

In code pleading. New matter constituting a defense ; new matter which, assuming the complaint to be true, constitutes a defense to it. Carter v. Eighth Ward Bank, 33 Misc. Rep. 128, 07 N. Y. Supp. 300.