Definition:
Games 2.0 refers to new browser-based games that rely on Web 2.0 techniques like Ajax or run on a social networking platform. Some of these games are web mashups that use a variety of sources to create the game, like Flicktionary's use of Flickr to create a Pictionary-type game.
Facebook in particular is becoming a major platform for social gaming with the Facebook Developers Platform and the wide audience of Facebook users.