An affirmative defense requires two things to be true:
- The defendant does not deny the allegations of the plaintiff, hence the term affirmative. The defendant affirms that the plaintff's claims are true.
- The defendant must present new evidence to support the affirmative defense to the court that if true would be "legally sufficient to excuse the defendant."
In a civil case, the burden of proof lies with the plaintiff, not with the defendant. Yet a defendant who is affirmatively defending their action takes on the burden to prove their affirmative evidence "credible.".
In civil cases, the plaintiff has the burden of proving his case by a preponderance of the evidence. (Cornell Law School)
That said, the burden of proof may shift to the defendant if the defendant raises a factual issue in defense to the plaintiff's claims. (Cornell Law School about Nolo's Plain-English Law Dictionary)