trea·tise /ˈtritəs ||təz/
1. A written composition on a particular subject, in which its principles are discussed or explained; a tract.
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.]
n : a formal exposition