Petitioner

A petitioner is a very specific legal term. It is the person who presents a formal legal petition to a court or a legislature that requests a certain action be taken. A petitioner is also the person who files a motion or an appeal to a higher court. A party who petitions the Supreme Court to review a case is known as either the petitioner or the appellant. The person or entity (such as a corporation or government) who opposes the petition is called a respondent. In some cases a third party, an "amicus curiae" (friend of the court), may be heard.
Appeal process.
In the case of an appeal, the petitioner is usually the party who lost in a lower court. This can be either the plaintiff or the defendant who files the petition to the higher court for judicial review. If the petition is granted, the petitioner files an appellate brief giving the legal arguments for filing the appeal. The respondent is given a short period of time to respond to the brief with arguments for their side of the case and to refute any arguments made by the petitioner. In some jurisdictions, the petitioner is allowed to file a short reply. In all three cases, arguments must be based on the trial record which includes the Transcripts of testimony and evidence from the original trial.
Civil law.
In civil law the person who files the case against another person is called the petitioner or plaintiff. In a divorce this is the party who files or starts the action to legally end the marriage.