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).
In some jurisdictions the commencement of a lawsuit is done by filing a summons, claim form and/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(s) according to the rules of civil procedure.
Not all lawsuits are plenary actions, involving a full trial on the merits of the case. There are also simplified procedures, often called proceedings, in which the parties are termed petitioner instead of plaintiff, and respondent instead of defendant. There are also cases that do not technically involve two sides, such as petitions for specific statutory relief that require judicial approval; in those cases there are no respondents, just a petitioner.
The party to whom the complaint is against is the defendant; or in the case of a petition, a respondent. Case names are usually given with the plaintiff first, as in Plaintiff v. Defendant.