A lawsuit is a proceeding by a party or parties against another in the civil court of law.The archaic term "suit in law" is found in only a small number of laws still in effect today. The term "lawsuit" is used in reference to a civil action brought by a plaintiff a party who claims to have incurred loss as a result of a defendant's actions demands a legal or equitable remedy from a court.

A lawsuit may involve dispute resolution of private law issues between individuals, business entities or non-profit organizations. A lawsuit may also enable the state to be treated as if it were a private party in a civil case, as plaintiff, or defendant regarding an injury, or may provide the state with a civil cause of action to enforce certain laws.

Rules of criminal or civil procedure govern the conduct of a lawsuit in the common law adversarial system of dispute resolution. Procedural rules are constrained and informed by separate statutory laws, case laws, and constitutional provisions that define the rights of the parties to a lawsuit though the rules generally reflect this legal context on their face. The details of the procedure differ greatly from jurisdiction to jurisdiction, and often from court to court even within the same jurisdiction. These rules of the particular procedures are very important for litigants to know, because the litigants are the ones who dictate the timing and progression of the lawsuit.