(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."