Siobhan Thomas has recently asked the londongamesresearch group for people's definition-in-use of games. Being too lazy, vain and arrogant to do a serious search of the literature, I will make up my own definition and post it on my blog:
An activity is a game to the extent it emulates life with controlled risk.
And just to pre-empt the inevitable post-Wittgensteinien argument, this isn't the definition of game. This isn't even a definition of game. It's my definition of game, a language game that I'm playing with you.