Robin Torres, a columnist over at our sister site WoW.com has posted a fascinating piece trying to define casual. While it's WoW-themed, it could easily be adapted for any -- and every -- MMO out there and is well worth a read.
Given the addictive nature of MMOs, the word 'casual' might well seem an odd word to be attached to this genre of games. However Robin's definitions encompass every kind of gamer. Personally I find myself in the 'players with lives' section and I'm glad to be there. Head on over and check it out to find out where you belong.