Have you ever found yourself in the middle of an ongoing game, to which you were uninvited and unwelcome? I guess that you recall the incident as a moment, during which you were an intruder in a strange world. After some minutes, when the game was over, the very same place was again a part of your universe, a place you were welcome and free to move into. If you have had such an experience, then you know how the magic circle of a game feels like.
Seen from the inside, the magic circle of a game is the timespace that is totally dedicated to the game, this temporary world where you can focus on your gaming, thinking of nothing else. Playing a game means in fact placing yourself into the game’s timespace, this magic circle.