A claim by a defendant for damages against a plaintiff, for example, where a defendant in a patent suit counterclaims that the plaintiff is also infringing the defendant’s patents. See Compulsory Counterclaim.
Importantly, in patent and other intellectual property cases defences can often also be raised as counterclaims. Thus, for example, in a patent case the defendant can counterclaim for a declaratory judgment of invalidity or non-infringement. This can be particularly useful when an infringement claim is badly prepared and identifiable key elements of the patent claims are not present, either literally or or their equivalent. In such a situation the defendant can recite the absence of the element as a separate assertion, forcing the plaintiff to deny that recitation at risk of for example Rule 11, or alternately face a motion to dismiss.