Q.E.D. stands for quod erat demonstrandum, which means "which was to be demonstrated". I first came across it in high school mathematics, when we were learning formal geometry proofs. Once you've completed a mathematical proof, you stick the letters "Q.E.D." at the bottom, to show that you've proven that "which was to be demonstrated".

Interestingly, this Latin abbreviation appears to be on a downturn in popularity, replaced by a solid black square symbol, called a tombstone for no apparent reason.