Treat"er (?), n. One who treats; one who handles, or discourses on, a
subject; also, one who entertains.
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.
A treatise implies more form and method than an essay, but may fall
short of the fullness and completeness of a systematic exposition.