QED

Short for the Latin phrase "quod erat demonstrandum" meaning "that which was to be demonstrated"

Used at the end of a proof to show it is completed.

Also written Q.E.D.

Example: If m is an even integer, then m² is even
Proof: By definition of an even integer, there exists an integer n such that m = 2n
Thus we get m² = (2n)² = 2(2n²)
and we have m² is also even. QED

(For fun people say it is "Quite Easily Done".)