A more thorough explanation:

Definition: QED is an abbreviation of the Latin phrase "quod erat demonstrandum," which means "which was to be demonstrated or proved."

Example: After a long and complex mathematical proof, the mathematician wrote "QED" at the end to indicate that the proof was complete and the statement had been demonstrated.

Explanation: QED is often used in mathematics and other fields to indicate the end of a proof or demonstration. It is a way of saying "this is what I set out to prove, and I have successfully demonstrated it." The use of QED is a way of signaling to the reader that the argument is complete and that the conclusion is now established beyond doubt.