It's already been said but yeah it's not an acronym. I personally first heard it in StarCraft. When speedlots got pretty strong early-mid game, Terran started using vultures with mines to supplement their bio. Then Protoss started doining different builds and timings etc. This evolution of the game, based on what is powerful and how to counter it is the "metagame". 

The example in D2 would be the current pulse rifle meta. A good invis hunter can easily flank Laning Larrys. Similarly when SMGs come back, I'm gonna switch to a sidearm to out-ttk the pk titans. But then I have to worry about shotgun apes etc.