Both terms are obviously meaningless.

At the DICE convention/conference last week (the week before last?) David Crane (Pitfall) and David Jaffe (God of War) were on a panel about casual and downloadable games. They remarked that the majority of the games in the 1970's and '80's were actually casual because they were targeted (and succeeded in attracting) people from all age and gender groups outside of teenage and young adult males. I remember when I was young, it was my Dad who got me into the NES, and when he wasn't there, my mom could get me into a game of Mario and play some of it with me. Neither of them could be bothered to play games today.

At the same time, the term "hardcore" gamer is a joke. 10 years ago, I thought a "hardcore" gamer was someone heavily involved in the mod scene, who could set up and run their own server on Half-Life/Counter-Strike with bots and a custom map list. Someone who played or made fan translations of unreleased JRPG's was hardcore (like Kentarou said).

Saying MLG gamers are hardcore is an even funnier joke. MLG and GameBattles gained popularity with Halo and Call of Duty, who's aim assist is so blatant it compromises the ethos of their multiplayer modes (When your cross-hair is moving on its own on an enemy who's behind cover, you're not playing a great competitive game anymore). CAL and OGL are what I would consider real "hardcore" professional gamers.

The mainstream gaming media would have you believe someone who plays SmackDown vs. Raw, Halo, Madden, and Call of Duty is a "hardcore" gamer. I actually like those games, but it is hard for me to imagine that they aren't as casual as it gets. People like to term Wii owners and Facebook gamers as casual gamers. They're not - they're not "gamers" to begin with. They're people who consume various different mediums, some of books, some of movies, some of music, and some of games. The Wii is only as popular as it is because of the "Oprah effect". Once that group finds the newest exercise routine, or cooking appliance, or mainstream author, their Wiis will be in the dumpster next to their Twilight books and Michael Moore documentaries.

Of course some people want you to believe that games have finally broken into the mainstream after years of oppression (because their salaries depend on it).

But, again, there were "casual" demographics in the 1970s and 80s....