I'd simply define hardcore in terms of number of hours spent playing video games. 20 hours per week is a decent baseline. I suppose you could argue that number up or down if it doesn't suit your sensibilities.
Yes, that means you could be a hardcore gamer if you played 20 hours of Gems a week. C'mon, if you play that much Gems, you're pretty hardcore about it.
A hardcore game? I guess that would be one which is very challenging to master. One with a lot of depth so you could pour a lot of hours into it and become very skilled. Chess is about as hardcore as they come.
"The worst part about these reviews is they are [subjective]--and their scores often depend on how drunk you got the media at a Street Fighter event." — Mona Hamilton, Capcom Senior VP of Marketing
*Image indefinitely borrowed from BrainBoxLtd without his consent.
