A counterclaim is a claim that a defendant may have against a plaintiff. It may or may not arise out of the same transaction that gave rise to the plaintiff's claim. If it arises out of the same transaction as plaintiff''s original claim it is called a compulsory counterclaim. If the defendant fails to sue on it now he loses his right to sue on it in the future.
If it does not arise out of the transaction which gave rise to the original lawsuit, the counterclaim is permissive, and failure to assert the counterclaim at this time does not waive the defendant's right to sue on it in the future. If the counterclaim is compulsory, it falls within the ancillary jurisdiction of the court and may be heard regardless of the amount. A permissive counterclaim on the other hand must have an independent jurisdictional basis and therefore meet the jurisdictional amount requirement.