Discourse, treatise, disquisition, dissertation, thesis, monograph designate in common a systematic, serious, and often learned consideration of a subject or topic.
Discourse, the widest of these terms, may refer to something written or spoken but, since it fundamentally implies a passing from one link in a chain of reasoning to another, always suggests a careful formulation and usually a plan made in advance of expression.
Treatise implies a written work and suggests a formal, methodical, and more or less extended treatment, usually expository but sometimes argumentative or narrative; it often differs from discourse in not emphasizing reasoning and in referring to a lengthy work.
Disquisition stresses limitation of a subject and its investigation and discussion in writing; it carries no suggestion of failure or of success but throws its emphasis upon the exploratory nature of the discussion.
Dissertation presupposes examination and often independent examination of a subject and its discussion at length, usually in writing; it often denotes a treatise dependent on individual research by a candidate for a higher academic degree.
Thesis basically denotes a proposition which a person (as a candidate for an academic degree) advances and offers to maintain but is also often used interchangeably with dissertation.
Although some restrict it to a dissertation or other work (as one incorporating the results of a series of experiments) intended to maintain or prove a proposition laid down or clearly stated.
In practice, however, it may be difficult to tell whether the proposition or its treatment is in the user’s mind, so inextricably are the two notions intertwined.
Monograph implies a learned treatise on a single topic (as a particular biological species, a clearly restricted literary genre, or an author).
It typically refers to a work of this character published in a learned journal or as a pamphlet or small book.