treatise
n.a long and serious piece of writing that explains a subject in great detail. It is usually more formal and deeper than a simple essay.
n.a formal and systematic written discourse on a specific subject, generally longer and more exhaustive than an essay. It aims to present a thorough investigation of principles or facts.
“He wrote a long treatise on the history of local architecture.”
“The professor published a definitive treatise on international law that became a standard text for graduate students.”
“The philosopher's final work was a dense treatise exploring the intersection of ethics and aesthetics, challenging centuries of established thought on the nature of beauty.”
Typically paired with the preposition 'on' to indicate the subject matter.
From Middle English tretys, from Anglo-Norman tretiz and Old French traitis (“treatise, account”), from traitier (“to deal with, treat”).
Etymology adapted from Wiktionary, available under CC BY-SA 4.0.