A treatise is a long, serious piece of writing that works through one subject carefully and completely — a book or a long article, written by someone who wants to lay out everything they know and argue it properly. It is not a report, not an essay you write for homework, not a news article. The feeling of the word is: thick, formal, old, academic. Famous ones have this word in their titles — Hume's 'A Treatise of Human Nature'. Two things to notice about how people actually use it. First, it belongs to the library, not to conversation; you will meet it far more in reading than in speech. Second — and this is the fun part — modern speakers most often use it as a small joke. If a friend sends you a very long message explaining why one restaurant is better than another, you can say "thanks for the treatise". That teasing use is now so common that some people meet the joke before they meet the real word. The joke only works because the real word is so heavy. Note also that it is pronounced with the 's' as a soft sound at the end and is completely unrelated to a peace agreement, which is 'treaty' — different word, different job.