The plaintiff, claimant, or complainant is the party initiating a lawsuit, (also known as an action). The plaintiff is the one who is claiming damages for injury; or who is seeking redress of a wrong. In some jurisdictions the commencement of a lawsuit is done by filing a summons 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. 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 requires 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.