quod erat demonstrandum (Q.E.D.)
Literal translation: that which was to be proved
In an English sentence: The student who put his work on the board wrote quod erat demonstrandum beside the answer.
You might put this phrase or its abbreviation at the end of a math problem to show that your final answer is correct, and that you have proved what the problem required you to prove.
From Elizabeth Heimbach's book Latin Everywhere, Everyday