● Discussion, debate. A spoken or written discourse upon or treatment of a subject, in which it is discussed at length; a treatise, sermon, or the like.
--Oxford English Dictionary
（A treatise is a formal and systematic written discourse on some subject, generally longer and treating it in greater depth than an essay, and more concerned with investigating or exposing the principles of the subject.--wiki）
（A sermon addresses a theological, religious, or moral topic, usually expounding on a type of belief, law or behavior within both past and present contexts. Elements of preaching include exposition, exhortation and practical application.--wiki）

● A dissertation is a long formal piece of writing on a particular subject, especially for a university degree.
--Collins Essential English Dictionary

● wiki defines thesis
A thesis or dissertation is a document submitted in support of candidature for an academic degree or professional qualification presenting the author's research and findings.

● 6 steps to master's thesis writing
6 practical steps to help you focus your work on writing a master's thesis

● Yale's guide to thesis writing
Writing a thesis or dissertation is perhaps the most daunting part of graduate education. A thesis or dissertation marks the culmination of thousands of hours of training, research, and writing, and it represents you for years after graduation.