What disqualifies war from being a true game is probably what also disqualifies the stock market and business – the rules are not fully known nor accepted by all the players.
Marshall McLuhan – Understanding Media (via dropouthangoutspaceout)
This remains accurate for the domain of game theory proposed by economists and mathematicians up to the year 2000, approximately. I would also include rational selfish actors within this definition. More recently economists have tried to develop an “irrational” actor theory. There is also the emerging definition within game studies and learning games that includes the possibility that the game teaches its own rule set. That is to say that there exists the “novice player” for whom the rule set and the possibility space are both unclear – or unknown – and that through the exploration, through the play, the rule set reveals itself. This may be more dramatically realized in videogames, but can be also observed on playgrounds.