Definitions for "Powergaming"
"Tweaking" your character's skills and limitations to enhance potential character power level, instead of for roleplaying benefit. Also, playing only to collect more magic items and become more powerful than anyone else is powergaming. While being powerful is a benefit to your character, it should be secondary to enjoying yourself and properly acting out your character.
This is when a player insists on calling actions for someone else's character. This practice is not conducive to cooperative role-playing.
In role-playing games, powergaming is a particular way of playing in which the emphasis lies on developing a player character that is as powerful as possible.