A defense raised in a responsive pleading (answer) relating a new matter as a defense to the complaint; affirmative defenses might include contributory negligence or estopped in civil actions; in criminal cases insanity, duress, or self-defense might be used.
Source: U.S. Courts
When a defendant or person responding to a civil case has a reason that would make him or her "not guilty" or not at fault and gives the court new evidence to prove that. The defense has to prove what it says (called "burden of proof"). The defense has to explain this defense in their answer.
Source: California Courts.