"What was to be demonstrated" needed to be demonstrated!

A student in my office hours today asked me what the term QED meant at the bottom of a page, and got a (very short) lesson in Latin. Quod erat demonstrandum, "what was to be demonstrated", is a translation of the Greek hoper edei deixai used by Euclid to close a proof. Modern mathematical publications often substitute other symbols, including a  or simply note: proven.