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, in the United States, those listed in Rule 8 (c) of the Federal Rules of Civil Procedure. In criminal prosecutions, examples of affirmative defenses are self defense, insanity, entrapment and the statute of limitations.

In an affirmative defense, the defendant may concede that they committed the alleged acts, but they prove other facts which, under the law, either justify or excuse their otherwise wrongful actions, or otherwise overcomes 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 he can demonstrate that he had an honest and reasonable belief that another's use of force was unlawful and that the defendant's conduct was necessary to protect himself or someone else.

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.^{[citation needed]}

Among the most controversial affirmative defenses is the insanity defense, whereby a criminal defendant seeks to be excused from criminal liability on the ground that a mental illness, at the time of the alleged crime, prevented him or her from understanding the wrongful nature of his or her actions.

Because an affirmative defense requires an assertion of facts beyond those claimed by the plaintiff, generally the party who offers an affirmative defense bears the burden of proof. The standard of proof is typically lower than beyond a reasonable doubt. It can either be proved by clear and convincing evidence or by a preponderance of the evidence.

In this respect, affirmative defenses differ from ordinary defenses—claim of right, alibi, infancy, necessity, and self-defense (which is an affirmative defense at common law)—in which the prosecution has the burden of disproving beyond a reasonable doubt.

Rule 8 of the Federal Rules of Civil Procedure governs the assertion of affirmative defenses in civil cases that are 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, 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."