"Casual" is an awful term because it is being used to describe both a person's dedication to gaming as well as their preference of what kind of games they enjoy ...
Big games like Grand Theft Auto, Halo, and Mario all attract people who are "Fair Weather" gamers (for lack of a better term), who buy a system and only play a couple of games in the entire generation; in this way they're "Casual" in how dedicated these gamers are towards gaming.
Another way "Casual" is used is to describe a type of game a person is not typically interested in; this is both used to describe unconventional games like Wii Fit, as well as conventional games like Mario Kart, and is usually used in a negative way because "Real" gamers shouldn't be interested in this type of game.
