For me a casual game is a game I can sit down and have fun with for like 10 minutes or something! It´s not like with some games as for example Final Fantasy that you need a lot of time to play. If I have just half an hour or something like that, I don´t want to start up games like FF because that is a game I need more time to play. So my meaning is that a casual game is something you don´t need so much time to sit down with.
But thats just my opinion!