RUINER ACTUAL said:
Children don't play it, or there's no multiplayer.
I would actually argue that some of the most hardcore games are hardcore only for multiplayer. Counterstrike is definitely a hardcore game because you can't just pick it up and play it.
Starcraft must qualify, because you could make a career out of winning at video games, that seems hard core to me.
Some old school single player stuff might be hardcore, like Fallout 2 or Deus Ex, but come on. Children don't play Assassin's Creed and it has no multiplayer. I hardly think of it as hardcore (though very good). Batman Arkham Asylum is one of my favorite games, really wouldn't call it hardcore.
I think hardcore is how much effort you have to put into it to be considered good. Most modern single-player only games, despite being well designed, don't really require you to try that hard.