An affirmative defense is a category of defense used in litigation between private parties in common law jurisdictions, or, more familiarly, a type of defense raised in criminal law by the defendant. Affirmative defenses operate to limit or excuse or avoid a defendant's criminal culpability or civil liability, even if the factual allegations of plaintiff's claim are admitted or proven.
A clear illustration of an affirmative defense is self-defense. In its simplest form, a criminal defendant may be exonerated, if he can demonstrate that he had an honest and reasonable belief that his conduct was necessary to protect himself against another's use of unlawful force.
Among the most controversial affirmative defenses is the insanity defense, whereby a criminal defendant, shown to be insane at the time of their crime, seeks commitment to a mental institution in lieu of imprisonment.
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. What constitutes timely assertion is often itself the subject of contentious litigation.
Because an affirmative defense requires an assertion of facts beyond those claimed by the plaintiff, generally the party making an affirmative defense bears the burden of proof. The burden of proof is typically lower than beyond a reasonable doubt. It can either be proof by clear and convincing evidence or a preponderance of the evidence.
Rule 8 of the Federal Rules of Civil Procedure governs the assertion of affirmative defenses in civil cases filed in the United States district courts. Rule 8(c) specifically enumerates the following defenses: "accord and satisfaction, arbitration and award, assumption of risk, contributory negligence, discharge in bankruptcy, duress, estoppel, failure of consideration, fraud, illegality, injury by fellow servant, laches, license, payment, release, res judicata, statute of frauds, statute of limitations, waiver, and any other matter constituting an avoidance or affirmative defense."