Claimant

n. one who asserts a right or title to something

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. someone who claims a benefit or right or title; "claimants of unemployment compensation"; "he was a claimant to the throne"
(hypernym) applicant, applier
(hyponym) Pretender
(derivation) claim, lay claim, arrogate

(n.)
One who claims; one who asserts a right or title; a claimer.
  

A party to an explicit or implicit contract.