Quod erat demonstrandum is Latin for Which had to be proved
Quod erat demonstrandum, Q.E.D., QED Which was the thing to be demonstrated; Which was to be demonstrated. A formula appended at the end of a proof in geometry, or other mathematical solution, with the meaning, 'We have proven the proposition we set out to prove.'
(QED) proof/evidence has been provided as intended - this is the proof - (traditionally appended to a mathematical solution)
which was to be shown or demonstrated.