An abbreviation of the Latin phrase "quod erat demonstrandum". It literally translates as "which was to be demonstrated", and is a formal way of ending a mathematical, logical or physical proof. It's purpose is to alert the reader that the immediately previous statement, which naturally was arrived at by an unbroken chain of logic, was the original statement that we were trying to prove.
"I refuse to prove that I exist," says God, "for proof denies faith, and without faith I am nothing."
"But," say Man, "the Babel fish is a dead giveaway, isn't it? It could not have evolved by chance. It proves you exist, and so therefore, by your own arguments, you don't. QED."
"Oh dear," says God, "I hadn't thought of that" and promply vanishes in a puff of logic." -- Douglas Adams, from "A Hitchiker's Guide to the Galaxy"

Proof that x + 3 = 0 if x = -3

x + 3 = (-3) + 3
= 0
by redwind64 February 18, 2005
1) From the Latin 'quod erat demonstrandum', a mathematical term meaning that a proof is complete.

2) A mathematician's way of saying "OH SNAP BITCH!"
"So you see, by this simple proof, your theory is wrong and mine is correct. QED!"
by loudlylikethemouse January 30, 2005
Originally Latin meaning "quod erat demonstrandum" or "which was to be shown or proven", now used mainly by physics students to insult someone when something is proven wrong or false, typically with the words Mother Fucker added for effect.
Ha! Your wrong.. QED mother fucker!
by ChickensBitch October 27, 2003
"So you see, by this simple proof, your theory is wrong and mine is correct. QED, bitch!"
by MathmanX March 26, 2007
"And thus it follows that x=12 you dumb shit! Booyah! Errr... I meant QED!"
by ADHDfidget May 2, 2017
After proving something in mathematics, used to indicate that said proof is complete.
Prove that x+3=0 when x is -3...

x=-3 QED
by goatsmacker June 13, 2004
Quantum Electrodynamics: a branch of quantum theory describing the interaction of particles with photons. It's main proponenet was Richard Feynman, creator of the ever-popular 'Feynman diagrams.'
"quantum electrodynamics describes all physical phenomena except gravity, radioactivity and nuclear physics; there is no significant difference between experiment and theory" Feynman.
by DBell March 18, 2005