Anyone who has difficulty remembering the distinction between these words should recall that plaintiff is related to plaint and complaint (meaning "grievance") and complain ("to protest"). A plaintiff is a complainant, one who brings a suit in a court of law: "Marcia was the plaintiff' in this action." A defendant, as opposed to a plaintiff, is a defending party, a person sued or accused: "As plaintiff, you brought the action, and as defendant, I shall oppose you."
Dictionary of problem words and expressions. Harry Shaw. 1975.