Persistent Browser-Based Game. It's a game that's played on your internet browser, with no required downloads or special software, though it does save your progress.
Most pbbgs don't take that long to complete and are mainly used to kill time. Still, many people are addicted to them, because they can be played any time and there are so damn many of them.
Walt: Dude, did you finish your part of our presentation?
Dude: Damn, I forgot, I spent all night playing some pbbg on Armorgames.