A plaintiff is the party who initiates a lawsuit 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. "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".

न्यायशास्त्र में अभियोगी या मुद्दई (plaintiff) वह पक्ष होता है जो न्यायालय में मुकदमा लाकर किसी पर आरोप लगाता है। इसके विपरीत अभियुक्त (defendant) या बचाव पक्ष वह पक्ष होता है जिसपर आरोप लगया गया हो।