The Collaborative International Dictionary of English v.0.48:

Missive \Mis"sive\, a. [See Missive, n.]
   1. Specially sent; intended or prepared to be sent; as, a
      letter missive. --Ayliffe.
      [1913 Webster]

   2. Missile. "The missive weapons fly." --Dryden.
      [1913 Webster]

   Letters missive, letters conveying the permission, comand,
      or advice of a superior authority, as a sovereign. They
      are addressed and sent to some certain person or persons,
      and are distinguished from letters patent, which are
      addressed to the public.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Missive \Mis"sive\, n. [F. lettre missive. See Mission, n.]
   1. That which is sent; a writing containing a message.
      [1913 Webster]

   2. One who is sent; a messenger. [Obs.] --Shak.
      [1913 Webster]

WordNet (r) 3.0 (2006):

missive
    n 1: a written message addressed to a person or organization;
         "mailed an indignant letter to the editor" [syn: letter,
         missive]