The Collaborative International Dictionary of English v.0.48:

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.
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            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.
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   Note: A treatise implies more form and method than an essay,
         but may fall short of the fullness and completeness of
         a systematic exposition.
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   2. Story; discourse. [R.] --Shak.
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