Source : Webster's Revised Unabridged Dictionary (1913)

Treatise \Trea"tise\, n. [OE. tretis, OF. treitis, traitis, well
   made. See {Treat}.]
   1. A written composition on a particular subject, in which
      its principles are discussed or explained; a tract.
      --Chaucer.

            He published a treatise in which he maintained that
            a marriage between a member of the Church of England
            and a dissenter was a nullity.        --Macaulay.

   Note: A treatise implies more form and method than an essay,
         but may fall short of the fullness and completeness of
         a systematic exposition.

   2. Story; discourse. [R.] --Shak.

Source : WordNet®

treatise
     n : a formal exposition