1. One who spends a lot of time playing games.
2. One who takes the playing of games very seriously.
3. One who does well at playing a certain game.
4. (Derogatory) One who plays the game for their own advantage; often used by players of role playing games to refer to players who they believe care more about character advancement than about role playing.
1. Being in a long session of playing games.
Bob is a powergamer who plays 8 hours a day.
Bob is a powergamer who spends a lot of time planning about playing his game.
Bob is a Poker powergamer; he is really good at Poker.
Look at that 100th level Bob and his gear, man, is he a powergamer or what?
Bob is powergaming right now for 12 hours straight.
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 MUDs 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.
(online specific) Referring to a player of MMOs who goes to great lengths to maximize the amount of experience points (xp) they get in the smallest amount of time. They are obsessed with xp efficiency over all other things, including their own bladder status. Typical behaviors are inordinate amounts of time spent playing the game, getting the best equipment at all times for their character, and "farming" areas for the best loot and xp.
(typical powergamer quote) "Get out of my group. You'll ruin my XP."
A) A person who frustratingly attempts to win by all means at tabletop role-playing games.
B) A person who seeks to accomplish everything possible within a videogame - meet all secret goals, get every item, unlock every secret, etc.
A) "Last I heard, he had given himself a man-sized sword. What a powergamer."
B) "You bought and upgraded every 200 cars in that game? Man, you're a powergamer."