The best definition for hardcore that I can think of is 1 of 2 things, it's either:
1. A game that is amazing in quality, but isn't mainstream and rarely talked about from the average gamer and non-gamer.
2. A game that appeals to a lot of people, but the majority of the gamers that like it are of a single fan base.
This will only take a moment of your time. *steals your watch*