To me, a Hardcore gamer is someone whose primary hobby and pastime, is gaming. Not someone who only fires up Angry Birds while taking a dump. Someone to whom playing for 2 hours straight is "a good warmup."
I'd say anyone who plays more than a few hours a week, and they typically play more than just one genre of game. --- Asus P8Z68-V LE | Core i7 2600K | 8GB G.Skill Ripjaws DDR3 | Gigabyte GeForce GTX 660 Windforce OC PS3 | PS2 | PSP| Wii | 3DS | DS | X-Box 360 | X-Box | NES