Frogger on 19 January 2015
A core game is a game that requires a high level of gaming experience and knowledge. Journey is a core game because it's entire development is built around the idea that someone is at least intermediate when it comes to video game experience. Dark Souls is core because in order to play it successfully, you must have an intermediate experience with games.
Neither 3D World or GTA V are core games. They are casual games with core appeal. That's why most indies are core games. It's all about the expected knowledge level.
It has nothing to do with quality, though.
Most of the time, when people talk about "hardcore games," what they really mean is "western style games who's target audiences are people over 15.
...And JRPGs."
