Affirmative defense

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.

In an affirmative defense, the defendant may concede that she committed the alleged acts, but she proves other facts which, under the law, either justify or excuse her otherwise wrongful actions, or otherwise overcome the plaintiff's claim. In criminal law, an affirmative defense is sometimes called a justification or excuse defense. Consequently, affirmative defenses limit or excuse a defendant's criminal culpability or civil liability.

A clear illustration of an affirmative defense is self defense. In its simplest form, a criminal defendant may be exonerated if she can demonstrate that she had an honest and reasonable belief that another's use of force was unlawful and that the defendant's conduct was necessary to protect herself.

Most affirmative defenses must be pleaded in a timely manner by a defendant in order for the court to consider them, or else they are considered waived by the defendant's failure to assert them. The classic unwaivable affirmative defense is lack of subject-matter jurisdiction. The issue of timely assertion is often the subject of contentious litigation.

"Mistake of fact" is not an affirmative defense: it does not require proof but it does introduce doubt. In mistake-of-fact defenses, the defendant asserts that her mistaken belief prevents the establishment, beyond a reasonable doubt, of the required mens rea.

...
Wikipedia