Well I can say I have friends that plays more CoD than I play all my games together... I think he is a hardcore gamer... just because he plays only CoD didn't make it a worst gamer than me.
Well theres no real definition for the term hardcore, but I typically look at it as gaming enthusiats. The type who are well informed, egaged with the industry and passionate about the medium. But I think the term term casual/core gets thrown about to much. People should start using uniformed/informed, I think its a bit more specific when talking about people like this, who are probably the single largest demographic in the industry. Obviously playing very few games doesn't make you uniformed but I think its normaly telling of the range of games they're aware of.
I know people who only play sims but they love, also know a few grown men who only play pokemon lol. I don't anywould describe themselfs as a hardcore gamer.