A dissertation is a long piece of writing on one subject, written to earn a university degree. It is researched, argued, and much longer than a school essay. You live with one question for months or years and try to show you can answer it with evidence.

Though actually, British and American universities do not use the word the same way, and textbooks often hide that. In the United States, the big book for a PhD is usually a dissertation, and a master’s paper is often a thesis. In much of the UK and Europe, a dissertation is often the long paper for a master’s (sometimes even a bachelor’s), and the PhD book may be called a thesis. Always check what your own department means. Taiwanese students mostly say 論文 for both, and add 碩士 or 博士 when they need to be clear.