I used to consider myself a hardcore gamer. I used to play at least 2-3 hours a day in my younger days, starting with NES. One of my main goals was to be better than anybody else in games, and for the mast part I was. But as I grew older, I realized that there are other things in life other than video games. I have grown more into more of a casual gamer these days.
Overall, I would define hardcore gaming as not having much interest in other activities besides video games (having no life) as well as wanting to be better than everybody else.