Although I actually define a casual gamer as someone who plays linear games.
That's different from mine... I would have defined a casual gamer as someone who would look to a game while awaiting the pizza delivery, and then move on to a book, or a movie. Or (in the case of an addict), Someone that would play a game if it was straightforward enough, and rewarded them often. I don't see casual players as ones who would put too much stake in a game, or post on its forum, or monitor its development.
They might pick up a game that their friends play, and perhaps discard it for the new game that their friends play ~not giving the past one a second thought.
My definition for 'casual game' seems to have always been one that you can pick up for 10 minutes, and have a blast at it ~then quit for a few weeks, and suddenly pick it up where you left off, or just start again... You can't do this with Planescape or Baldur's gate; you can do it with Doom and heretic, and Zuma, and Bejeweled; and possibly some online games that don't decay while you are not playing.