It´s a word coming from latin "tractatus" and it is a literary form means essay, treatise or debate and discussion (in written form)on some subject. Often used for religious, political, cultural, moral/ethical or (natural)scientific subjects.
A "tractatus" is generally longer and treating it in greater depth than an essay, and more concerned with investigating or exposing the principles of the subject. (I´m a non-native speaker so I hope my explanation makes sense ;-) )

Hope it helps :-)