Somebody who enjoys regularly playing video games.
Any other stereotypical definition is useless: I play online games a lot. I am over 40, have a girlfriend, am in good physical shape and I hold down a fairly high-pressure job. I am college educated.
Some of the younger gamers that I meet online fit the nerdy stereotype but no more than the average cross-section of high-school students. There is a higher than average number of creative professionals in the game servers I go to (probably because I go to the Mac servers).
I also have LAN parties at my house - my friends come round, we drink a few beers, get a take-away, link our computers together and fight to the death. The other three are married and two have kids. Three of us have college degrees and we are all fairly high earners.
Your stereotype is way off.
My friends and I are serious gamers: we often meet to play Ghost Recon.