A lawsuit (or suit in law) is "a vernacular term for a suit, action, or cause instituted or depending between two private persons in the courts of law." The term refers to any proceeding by a party or parties against another in a court of law.

Sometimes, the term "lawsuit" is in reference to a civil action brought in a court of law in which a plaintiff, a party who claims to have incurred loss as a result of a defendant's actions, demands a legal or equitable remedy. The defendant is required to respond to the plaintiff's complaint. If the plaintiff is successful, judgment is in the plaintiff's favor, and a variety of court orders may be issued to enforce a right, award damages, or impose a temporary or permanent injunction to prevent an act or compel an act. A declaratory judgment may be issued to prevent future legal disputes.