What is an affirmative defense and how can it be used in a legal case?

An affirmative defense is a legal argument used by a defendant to counter or justify the claims made by the plaintiff. It is a proactive assertion of facts that, if proven, can excuse or justify the defendant's actions. Affirmative defenses can be used in a legal case to shift the burden of proof onto the plaintiff or to show that the defendant should not be held liable for the alleged wrongdoing.