A plaintiff (Π in legal shorthand) is the party who initiates a lawsuit (also known as an action) before a court. By doing so, the plaintiff seeks a legal remedy. If this search is successful, the court will issue judgment in favor of the plaintiff and make the appropriate court order (e.g., an order for damages). "Plaintiff" is the term used in civil cases in most English-speaking jurisdictions, the notable exceptions being England and Wales, where a plaintiff has, since the introduction of the Civil Procedure Rules in 1999, been known as a "claimant" and Scotland, where the party has always been known as the "pursuer". In criminal cases, the prosecutor brings the case against the defendant, but the key complaining party is often called the "complainant".

In some jurisdictions, a lawsuit is commenced by filing a summons, claim form or a complaint. These documents are known as pleadings, that set forth the alleged wrongs committed by the defendant or defendants with a demand for relief. In other jurisdictions, the action is commenced by service of legal process by delivery of these documents on the defendant by a process server; they are only filed with the court subsequently with an affidavit from the process server that they had been given to the defendant according to the rules of civil procedure.