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.