A defense is a denial, or answer or plea in opposition to the truth or validity of a claim by a plaintiff. It may be made by an oral motion, written pleading, or through the introduction of evidence. An affirmative defense is a type of defense in which the defendant seeks to avoid liability by introducing new evidence not addresses in the claims of the plaintiff's complaint. Such a defense must be raised in the defendan't answer and the defendant has the burden of proof of proving the defense.
There are also equitable defenses, which appeal to basic notions of equity and fairness. Equitable defenses may include illegality, such as a contract for purchase of illegal goods, or fraud. Another example is defendant's claim that the plaintiff has "unclean hands", meaning that the plaintiff acted in a malicious or intentionally wrongful manner in regard to the transactions that form the basis of plaintiff's claim.
