A petitioner is the party who asks a court for something. The term appears most often in appellate and Supreme Court practice. The petitioner files a petition for certiorari, asking the higher court to review a lower court's decision. The opposing party is the respondent.

In trial courts, the terminology varies. A plaintiff files a complaint. A petitioner files a petition. The difference is largely procedural. Petitions are common in family law, probate, and administrative appeals. A petitioner might ask for a protective order, a name change, or review of a government agency decision.

Petitioner versus other parties

• Petitioner: initiates the petition.
• Respondent: responds to the petition.
• Plaintiff: initiates a civil lawsuit.
• Defendant: defends against a civil or criminal claim.
• Appellant: appeals a lower court decision.

The label matters less than the role. A petitioner carries the burden of persuading the court that the requested action is warranted. In Supreme Court practice, the petitioner files a brief explaining why the Court should grant review. The respondent files an opposition. Four of the nine justices must vote to grant certiorari. Most petitions are denied, which means the lower court ruling stands. Being a petitioner is an uphill climb, especially at the highest levels.