What is the legal term for a justification that excuses criminal responsibility if proven?

The legal term for a justification that excuses criminal responsibility if proven is called an affirmative defense. This type of defense allows a defendant to fight charges against them by presenting evidence that, if accepted by the court, can negate legal liability for the crime in question.

Affirmative defenses require the defendant to provide proof of the justification, unlike typical defenses where the burden is primarily on the prosecution to prove their case. Common examples of affirmative defenses include self-defense and the insanity defense; however, not all defenses fall into this category as some simply contest the prosecution's claims without providing justifying circumstances.

In the context of criminal law, affirmative defenses play a critical role in ensuring that individuals can argue their case based on specific legal principles, aiming to avoid conviction even when the facts of the case indicate some level of wrongdoing.