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1.
A game published by Activision which is a simple "copy-and-paste" stand-alone game. The graphics and physics engine for this "new" and "improved" game are "phenomenal", with little to no improvement in comparison of the Black Ops engine. Many people are fooled by the official label of the game "Call of Duty: Modern Warfare 3", but most gamers recognize that the game was just half-assed and only minor adjustments were made.

Everybody knows that this game should have been a 40 USD expansion, the same way people thought Halo 3:ODST should have been an expansion for Halo 3 (original).

People often confuse this game with Modern Warfare 2. Hence, the name Modern Warfare 2.5
"Hey man, do you want to jump onto that MW3?"

"Naw dude, I'm getting on Battlefield 3 and then Halo bro! Screw that Modern Warfare 2.5!"

"Dawg, you right! I'm gonna play with you hold on real quick!"
by Basilman212 November 09, 2011
 
2.
The correct name for Modern Warfare 3. It is essentially Modern Warfare 2, but with new maps. Don't waste your money on this game.
Guy who just got MW3:

"OH yeah MW3! This will be soo awesome!!"

5 minutes later...

"WTF this is just like MW2! This game is more like Modern Warfare 2.5 what a rip-off!"
by iduncurr December 29, 2011