Affirmative Defense

A new fact or set of facts that operates to defeat a claim even if the facts supporting that claim are true.

A plaintiff sets forth a claim in a civil action by making statements in the document called the complaint. These statements must be sufficient to warrant relief from the court. The defendant responds to the plaintiff's claims by preparing an answer in which the defendant may deny the truth of the plaintiff's allegations or assert that there are additional facts that constitute a defense to the plaintiff's action. For example, a plaintiff may demand compensation for damage done to his or her vehicle in an automobile accident. Without denying responsibility for the accident, the defendant may claim to have an affirmative defense, such as the plain-tiff's contributory Negligence or expiration of the Statute of Limitations.
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affirmative defense
n. when a defendant files an answer, in addition to denying some or all of the allegations, he/she can state what are called "affirmative defenses." These defenses can contain allegations, take the initiative against statements of facts contrary to those stated in the original complaint against them, and include various defenses based on legal principles. Many of these defenses fall into the "boilerplate" (stated in routine, non-specific language) category, but one or more of the defenses may help the defendant.
http://legal-dictionary.thefreedictionary.com/Affirmative De...

Affirmative Defense

A defense in which the defendant introduces evidence, which, if found to be credible, will negate criminal or civil liability, even if it is proven that the defendant committed the alleged acts. Self-defense, entrapment, insanity, and necessity are some examples of affirmative defenses.
https://www.law.cornell.edu/wex/affirmative_defense

http://legaldictionary.net/affirmative-defense/
https://en.wikipedia.org/wiki/Affirmative_defense