An affirmative defense is a defense used in litigation between private parties in common law jurisdictions. Affirmative defenses work to limit or excuse a defendant's liability even if the plaintiff's claim is proven, based on facts outside those claimed by the plaintiff.
An affirmative defense must be timely pleaded by the defendant in order for the court to consider it, or else it is considered waived by the defendant's failure to assert it. Because affirmative defenses require the assertion of facts beyond those claimed by the plaintiff, generally the party pleading an affirmative defense has the burden of proof on that defense. The burden of proof is typically lower than beyond a reasonable doubt. It can either be clear and convincing or preponderance of the evidence.
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