a: a formal written document; specifically: a legal instrument in epistolary form issued under seal in the name of the English monarch
b: an order or mandatory process in writing issued in the name of the sovereign or of a court or judicial officer commanding the person to whom it is directed to perform or refrain from performing an act specified therein <writ of detinue><writ of entry><writ of execution>
c: the power and authority of the issuer of such a written order —usually used with run<outside the United States where … our writ does not run — Dean Acheson>
In common law, an order issued in the name of a sovereign or court commanding a person to perform or refrain from performing a specified act. It was a vital official instrument in Old English law. A plaintiff would commence a suit by choosing the proper form of action and obtaining a writ appropriate to the remedy sought; its issuance forced the defendant to comply or to appear in court. Writs were also constantly in use for financial and political purposes of government. Though the writ no longer governs civil pleading and has lost many of its applications, the extraordinary writs, especially of habeas corpus, mandamus (commanding the performance of a ministerial act), prohibition (commanding an inferior court to stay within its jurisdiction), and certiorari, reflect its historical importance as an instrument of judicial authority.