A player of a game that seeks to use the system to dominate and frustrate other players.
This is typically not a problem when dealing with games that encourage competitive playing, but swiftly becomes a problem when the objective of the game is not to defeat your opponenet, either real or perceived, but to co-operate and socialize with other players.
As such, power-gamers are the bane of certain role-playing games, social games and online games like MUD
s and their ilk, and games where not everyone wants to make you their bitch.
However, certain games suit powergamers well -- games like Monopoly and the (now discontinued) Paranoia RPG setting by West End Games.
Well, yeah, Fabian is a power-gamer... he's a wonderful person to be with, and very accommodating, but never, ever, play against him -- you'll just end up hating him.