A petitioner is a person who pleads with governmental institution for a legal remedy or a redress of grievances, through use of a petition.
[1] The petitioner may seek a legal remedy if the state or another private person has acted unlawfully.
In this case, the petitioner, often called a plaintiff, will submit a plea to a court to resolve the dispute.
A group of 17th century English politicians became known as Petitioners, due to their support of the Exclusion Bill, a bill which would prevent the succession to the throne of the Catholic James, Duke of York, the heir apparent of King Charles II.
In the heat of the dispute, the two factions traded insulting epithets; with the result that the Petitioners became known as the Whigs and their opponents as Tories.