Petitioner

n. one who submits a request; one who pleads, one who appeals; one who submits a legal demand

A plaintiff (Π in legal shorthand), also known as a claimant or complainant, is the party who initiates a lawsuit (also known as an action) before a court. By doing so, the plaintiff seeks a legal remedy, and if successful, the court will issue judgment in favor of the plaintiff and make the appropriate court order (eg. an order for damages).
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Noun
1. one praying humbly for something; "a suppliant for her favors"
(synonym) suppliant, supplicant
(hypernym) applicant, applier
(hyponym) besieger
2. someone who petitions a court for redress of a grievance or recovery of a right
(synonym) suer
(hypernym) plaintiff, complainant
(derivation) petition

(n.)
One who presents a petition.
  

A person who presents the petition