Русско-англо-русский юридический словарь

In American English, a dissertation is a research paper that’s required to earn a doctorate degree, while a thesis is a research paper required to earn a master’s degree. (Grammarly)

In countries where the academic system is based on the British system of university education, ‘dissertation’ refers to the body of work at the end of an undergraduate or master’s degree, while ‘thesis’ refers to the body of work produced at the end of a PhD. In countries and institutions following the American system of education, the terms tend to be used in reverse. (Charlesworth Author Services)