Wiktionary
n. (context legal English) A defense against a suit or criminal charge that asserts mitigate facts rather than contesting the main fact of the allegation.
Wikipedia
An affirmative defense to a civil lawsuit or criminal charge is a fact or set of facts other than those alleged by the plaintiff or prosecutor which, if proven by the defendant, defeats or mitigates the legal consequences of the defendant's otherwise unlawful conduct. In civil lawsuits, affirmative defenses include the statute of limitations, the statute of frauds, waiver, and other affirmative defenses such as those listed in Rule 8 (c) of the Federal Rules of Civil Procedure. In criminal prosecutions, examples of affirmative defenses are self defense, insanity, and the statute of limitations.
Usage examples of "affirmative defense".
Again we concur, as we do also with the dismissal of plaintiff's Third Cause of Action alleging fraud on the part of Kiester in his repeated changes of name to conceal his original access to the play under his original identity, since we find in defendant's Third Affirmative Defense of these name changes tor professional reasons' expressed in his own indelicate choice of words 'I'm a Jew the minute I step off the plane in LA.
The burden of proof is going to shift once I start to build up an affirmative defense.