A player who spends a very large number of hours playing per week.
A player particularly interested in challenge, difficult content, or 'beating the game'.
Hardcore players are usually but not necessarily raiders. The very elite of hardcore gamers are sometimes called the bleeding edge. These are the players who attempt to be first to not only encounter content, but to be the first to devise strategies to overcome it.
The 'casual' and the 'hardcore' are often considered to be at opposite ends of the gaming spectrum, and much as been written about the casual player whining to make games easier and the 'bleeding edge' hardcore player rampaging through content and then crying they have nothing to do. Much forum angst is spent on this issue.