My definition stems from the actual definition of casual (imagine that): basically a game that is designed in such a manner that it does not have to be taken serious to enjoy the intended experience. Such games include all the games listed in the OP (Moster Hunter is arguable) and many more simply because they are designed to appeal to a wide audience. In order to do this they have to be accessible and fit many different lifestyles. Other games include: Halo, Call of Duty, Gears of War, God of War, Killzone, most shooters, most Nintendo games (a notable exception being Fire Emblem), and the majority of games in general.
Other games are designed inherently for people who are looking at games more for an experience (for lack of better words) than simply recreation and likely as a result demand more of the player in terms of time and effort involved. These are games such as RPGS (both Japanese and Western) and games that are rather difficult in nature.
The typical vagueness of the definition only lies from people taking the definition out of its scope to fit their intended purpose to essentially downgrade or defend the quality of the game. I do not feel casual games are inferior nor vice versa. They are not usually games that I enjoy simply because I'm a more serious person. Any core game can be enjoyed in a more casual manner and any casual game can still be played in a serious manner. Non-casual games can still sell well, though it is much more unlikely.
With that said, the argument should really be toward console manufacturers who decide to overly rely on casual at the expense of the core gamer. I understand that casual gamers are necessary for the success of the console; however, there really is no excuse for not diversifying the experiences that your console offers.
How do you breathe again?
