Webster:
Main Entry: trac·tate
Pronunciation: 'trak-"tAt
Function: noun
Etymology: Latin tractatus, from tractare to draw out, handle, treat -- more at TREAT
Date: 15th century
TREATISE, DISSERTATION
Treatise
Main Entry: trea·tise
Pronunciation: 'trE-t&s also -t&z
Function: noun
Etymology: Middle English tretis, from Anglo-French tretiz, from Old French traitier to treat
Date: 14th century
1 : a systematic exposition or argument in writing including a methodical discussion of the facts and principles involved and conclusions reached <a treatise on higher education>