I don't think it has to to with time or mindset as most people have suggested.

It has to do with how committed you are to other people.

For instance, a hardcore player when a new raid comes out is committed to playing 4 hours a day for a few nights each week. This creates an obligation to people and to the game.

Casual players don't have this obligation regardless of how many hours they play a week. They could play for 40 hours a week whenever they want and never feel as though they cant leave.

My definition, i believe, can be proven in game. Ever wonder why bad players are in full Coil gear? or why very good players with a lot of in game time have none or haven't run coil at all? one is committed and dedicated to a group of players and the other is not. (no offense to hardcore raiders, i know many of you are elite.)

Its not about how much you play or how much you want to win but how many hours you are willing to invest without the possibility of leaving.