The petitioner is the party who presents a petition or motion to the court. The term is frequently used to describe the appellant in an appeal, where the petitioner is usually the party who lost in the lower court. This can be either the plaintiff or defendant from the court below, as either of the parties can present the case to a higher court for further proceedings. The opposite of the petitioner is the respondent, against whom a petition or motion is filed.
The term petitioner is likely to be used instead of appellant in a variety of circumstances. Historically, courts of equity would use the term the plaintiff as the petitioner and the defendant as the respondent even prior to appeal. Rule 21 of the Federal Rules of Appellate Procedure uses the term petitioner when an appeal is taken based on a writ of mandamus or a writ of prohibition. Federal appellate courts also use the term petitioner when an appeal is taken from administrative agency adjudicatory proceedings. State courts vary on whether they use the term petitioner or appellant.
[Last updated in February of 2024 by the Wex Definitions Team]