I looked up on wiktionary and dictionary.com and found the same three definitons for "hardcore".
1. having an extreme dedication to a certain activity(wiktionary)/unswervingly committed; uncompromising; dedicated(dictionary.com)
Gaming Translation: This has nothing to do with games themselves. This has to do with the gamer and their dedication to the game's they play. Under this definition, all games can be hardcore, and the play is the determining factor.
2. Resistant to change(wiktionary)/being so without apparent change or remedy; chronic(dictionary.com)
Gaming Translation: Basically this means games that stay stubbonly the same even with technical advances in hardware. I suppose that would make Mega Man the most hardcore series out of the bunch and the entire SHMUPS genre. I'd never thought of this, but I like this definition of a hardcore game.
3. Obscene or explicit(wiktionary)/pruriently explicit; graphically depicted(dictionary.com)
Gaming Translation: This is the one everyone hates the most, but it's actually true. Sex and violence in games falls under hardcore. GTA, Manhunt, Conker's Bad Fur Day, Resident Evil, Fallout, Leisure Suit Larry, God of War, etc. would all fall under this category.
So there you go. Even though I thought it would, difficult games do not fall under the term hardcore. Then again neither do "fun" games either. Those are your definitions, now live with them and bask in defeat because those idiots screaming "GTA is bloody and has sex so it's hardcore" were actually right for once.