The Collaborative International Dictionary of English v.0.48:

Writ \Writ\, n. [AS. writ, gewrit. See Write.]
   [1913 Webster]
   1. That which is written; writing; scripture; -- applied
      especially to the Scriptures, or the books of the Old and
      New testaments; as, sacred writ. "Though in Holy Writ not
      named." --Milton.
      [1913 Webster]

            Then to his hands that writ he did betake,
            Which he disclosing read, thus as the paper spake.
                                                  --Spenser.
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            Babylon, so much spoken of in Holy Writ. --Knolles.
      [1913 Webster]

   2. (Law) An instrument in writing, under seal, in an
      epistolary form, issued from the proper authority,
      commanding the performance or nonperformance of some act
      by the person to whom it is directed; as, a writ of entry,
      of error, of execution, of injunction, of mandamus, of
      return, of summons, and the like.
      [1913 Webster]

   Note: Writs are usually witnessed, or tested, in the name of
         the chief justice or principal judge of the court out
         of which they are issued; and those directed to a
         sheriff, or other ministerial officer, require him to
         return them on a day specified. In former English law
         and practice, writs in civil cases were either original
         or judicial; the former were issued out of the Court of
         Chancery, under the great seal, for the summoning of a
         defendant to appear, and were granted before the suit
         began and in order to begin the same; the latter were
         issued out of the court where the original was
         returned, after the suit was begun and during the
         pendency of it. Tomlins. Brande. Encyc. Brit. The term
         writ is supposed by Mr. Reeves to have been derived
         from the fact of these formulae having always been
         expressed in writing, being, in this respect,
         distinguished from the other proceedings in the ancient
         action, which were conducted orally.
         [1913 Webster]

   Writ of account, Writ of capias, etc. See under
      Account, Capias, etc.

   Service of a writ. See under Service.
      [1913 Webster]