A subpoena (pronounced 'suh-pee-nah') is a writ commanding a person to appear under penalty (from the Latin).
It is used to compel the testimony of witnesses in a trial. Usually it can be issued by a judge or by the lawyer representing the plaintiff or the defendant in a civil trial or by the prosecutor or the defence attorney in a criminal proceeding. The United States Congress also has the power to issue subpoenas and can punish individuals who fail to comply by contempt of Congress, which is similar to contempt of court. It has its source in British common law and it is now used almost with universal application throughout the Anglo-American common law world.
