Dictionary.net

Sometimes called a cross-claim, legal paperwork that a defendant files to initiate her own lawsuit against the original plaintiff, a co-defendant or someone who is not yet a party to the lawsuit. A cross-complaint must concern the same events that gave rise to the original lawsuit. For example, a defendant accused of causing an injury when she failed to stop at a red light might cross-complain against the mechanic who recently repaired her car, claiming that his negligence resulted in the brakes failing and, hence, that the accident was his fault. In some states where the defendant wishes to make a legal claim against the original plaintiff and no third party is claimed to be involved, a counterclaim, not a cross-complaint, should be used.