plaintiff

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A plaintiff is the party who initiates a lawsuit in a civil matter.

The plaintiff may be an individual or an entity. A plaintiff files a complaint with a court against the defendant, initiating the action. In bringing an action, a civil suit must be prosecuted in the name of real party in interest, so the plaintiff must be the person or a representative of the person who possesses a claim against the defendant.

A defendant can also be a third-party plaintiff in the same case when they serve a complaint on a non-party, the third-party defendant, under Federal Rule of Civil Procedure 14. In certain situations, a plaintiff can actually be added to a case under Federal Rule of Civil Procedure 19, when required joinder calls for the party to be joined on the side of the original plaintiff as a co-plaintiff. 

A plaintiff may also be referred to as a petitioner, especially in equity, although the term petitioner can also be applied to either the plaintiff or defendant on appeal, depending on who is presenting the case to the higher court for further proceedings.

[Last updated in February of 2024 by the Wex Definitions Team]