Counterclaim

In a court of law, a party's claim is a counterclaim if one party asserts claims in response to the claims of another. In other words, if a plaintiff initiates a lawsuit and a defendant responds to the lawsuit with claims of their own against the plaintiff, the defendant's claims are "counterclaims."

Examples of counterclaims include:
 * After a bank has sued a customer for an unpaid debt, the customer counterclaims (sues back) against the bank for fraud in procuring the debt. The court will sort out the different claims in one lawsuit (unless the claims are severed).
 * Two cars collide. After one person sues for damage to his/her car and personal injuries, the defendant counterclaims for similar property damage and personal injury claims.

United States
In U.S. federal courts, counterclaims can arise on various occasions, including e.g.:
 * an attempt by the defendant to offset or reduce the amount/implications of the plaintiff's claim;
 * a different claim by the defendant against the plaintiff;
 * a claim by an impleaded third-party defendant against the original defendant acting as a third-party plaintiff;
 * a claim by any party against another party who has made a crossclaim against them

Counterclaims v. crossclaims
Dependent upon the location of where the lawsuit was originated, the defending party has a period of time to file a countersuit, also known as a counterclaim, against the claiming party. This is a direct claim from the defending party against the party who initiated the lawsuit for concurrent claims, including being wrongfully sued.

A crossclaim is a pleading made against a party who is a co-defendant or co-plaintiff. A crossclaim is against anyone who is "on the same side of the lawsuit". An example of this is a manufacturing company who ships their product through a third-party transportation company to the buyer. Upon the products being inspected by the buyer, the buyer finds that the product has been damaged in shipping and refuses to pay. If the manufacturer sued the buyer, the buyer would serve an answer with a denial that the buyer owed money to the manufacturer and a crossclaim to the shipping company to compensate for the damages.

Compulsory v. permissive
Under the Federal Rules of Civil Procedure (FRCP), counterclaims are either compulsory or permissive.

Permissive counterclaims comprise "any claim that is not compulsory." Such claims may be brought, but no rights are waived if they are not. Courts rarely give permissive counterclaims the necessary supplemental jurisdiction to be brought.

A claim is a compulsory counterclaim if, at the time of serving the pleading,
 * 1) the counterclaim "arises out of the transaction or occurrence that is the subject matter of the opposing party's claim,"
 * 2) AND the counterclaim "does not require adding another party over whom the court cannot acquire jurisdiction,"
 * 3) AND "when the action was commenced, the [otherwise mandatory counterclaim] was [not] the subject of another pending action,"
 * AND
 * EITHER the opposing party sued on its claim by a process that established personal jurisdiction over the pleader on that claim, (i.e., NOT by a process such as attachment)
 * OR (if personal jurisdiction was not established over the pleader), the pleader asserts some other mandatory counterclaim.

This last (fourth) requirement is explained in the official notes as follows:

If the counterclaim is compulsory, it must be brought in the current action or it is waived and lost forever.

Various tests have been proposed for when a counterclaim arises from the same transaction or occurrence, including same issues of fact and law, use of the same evidence, and logical relation between the claims.