Pursuer

A plaintiff (Π in legal shorthand) 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 (e.g., an order for damages). "Plaintiff" is the term used in civil cases in most English-speaking jurisdictions, the notable exception being England and Wales, where a plaintiff has, since the introduction of the Civil Procedure Rules in 1999, been known as a "claimant", but that term also has other meanings. In criminal cases, the prosecutor brings the case against the defendant, but the key complaining party is often called the "complainant".

In some jurisdictions the commencement of a lawsuit is done by filing a summons, claim form or a complaint. These documents are known as pleadings, that set forth the alleged wrongs committed by the defendant or defendants with a demand for relief. In other jurisdictions the action is commenced by service of legal process by delivery of these documents on the defendant by a process server; they are only filed with the court subsequently with an affidavit from the process server that they had been given to the defendant according to the rules of civil procedure.

In most English speaking jurisdictions, including Hong Kong, Nigeria, Australia, Canada and the United States, as well as in both Northern Ireland and the Republic of Ireland, the legal term "plaintiff" is still in use, as a general term for the party taking action in a civil case.

...
Wikipedia