Someone who makes a legal complaint against someone else in court. (Cambridge Dictionary)
A person who brings a case against another in a court of law. (Oxford Dictionaries)
A person who brings a legal action. (Merriam – Webster)
A plaintiff is a person who brings a legal case against someone in a court of law. (Collins Dictionary)
Defendant’s Response & Motions in Civil Litigation.
Defendant vs. Plaintiff – What’s the Difference?