What is Writ?

Legal Definition
In common law, a writ is a formal written order issued by a body with administrative or judicial jurisdiction; in modern usage, this body is generally a court. Warrants, prerogative writs and subpoenas are common types of writ but many forms exist and have existed.

In its earliest form a writ was simply a written order made by the English monarch to a specified person to undertake a specified action; for example, in the feudal era a military summons by the king to one of his tenants-in-chief to appear dressed for battle with retinue at a certain place and time. An early usage survives in the United Kingdom and Canada in a writ of election, which is a written order issued on behalf of the monarch (in Canada, the Governor General) to local officials (High Sheriffs of every county in the historical UK) to hold a general election. Writs were used by the medieval English kings to summon persons to Parliament (then consisting of the House of Lords alone) whose advice was considered valuable or who were particularly influential, who were thereby deemed to have been created "barons by writ".
-- Wikipedia
Legal Definition
A writ is an order issued by a legal authority with administrative or juridicial powers, typically a court.

See Writ of certiorari, Writ of error, Writ of habeas corpus, Writ of mandamus
Legal Definition
Practice. A mandatory precept issued by the authority, and in the name of the sovereign or the state, for the purpose of compelling the defendant to do something therein mentioned.

2. It is issued by a court or other competent jurisdiction, and is return-able to the same. It is to be under seal and tested by the proper officer, and is directed to the sheriff, or other officer lawfully authorized to execute the same. Writs are divided into, 1. Original. 2. Of mesne process. 3. Of execution. Vide 3 Bl. Com. 273; 1 Tidd, Pr. 93; Gould on Pl. c. 2, s. 1. There are several kinds of writs, some of which are mentioned below.
-- Bouviers Law Dictionary
Legal Definition
A precept In writing, couched in the form of a letter, running in the name of the king, president or state, issuing from a court of justice, and sealed with its seal, addressed to a sheriff or other officer of the law or directly to the person whose action the court desires to command, either as the commencement of a suit or other proceeding or as incidental to its progress, and requiring the performance of a specified act or giving authority and commission to have it done. For the names and description of various particular writs, sce the following titles. In old English law. An instrument in the form of a letter; a letter or letters of attorney. This is a very ancient sense of the word. In the old books, "writ" is used as equivalent to "action;" hence writs are sometimes divided into real, personal, and mixed. In Scotch law. A writing; an instrument in writing, as a deed, bond, contract, etc. 2 Forb. Inst, pt 2, pp. 175-179.
-- Black's Law Dictionary
Legal Definition
Process in a civil action, a warrant in a criminal one. See 23 Conn. 238. See, also, Process; Warrant. See, also, many different sorts of writs beginning with the prefix De, following De admensuratione donis.
-- Ballentine's Law Dictionary