Compulsory counterclaimA compulsory counterclaim is a claim made by a defendant against a plaintiff that arises from the same transaction or occurrence as the plaintiff's claim. The claim is compulsory in this situation in that it must be raised in the defendant's answer, or it is waived. If defendant fails to assert counterclaim in his answer, he is thereafter precluded from asserting it against plaintiff in the plaintiff's pending action or in an independent action.

An exception to this rule exists when the pleader fails to make the compulsory counterclaim through oversight, inadvertence, or excusable neglect, or when justice requires-- then the pleader may by leave of court set up counterclaim by amendment.