A person that wants to take an advantage in the game for being stronger, or having strong friends. Usually this is done by threatening weaker players with the fact that the power abuser(s) will kill them if they don't do what they say. Or alternatively, another way is to trap a player with monsters while Healing the monster until the player dies on Optional PvP worlds or on Open PvP without getting an unjustified kill.
On Optional PvP worlds it usually doesn't happen in the same way because they can't kill you, unless by using the War System. However, due to the fact they are stronger they will sometimes steal experience or lootbags from you until you leave. One of the more common power abusing methods is to lure monster(s) (not in use as much since the 7.5 update).
On Hardcore PvP worlds, players will kill rather than to abuse power.
In the companies eyes, power abuse is an extreme cause for concern when a group of players make gaming-life impossible for other players. When this happens there is usually no control over this group of people and Gamemasters must step in and take action. In this scenario the group of players "rule" over the server and also the weaker players, the weaker players on that world would have no impact on removing this group of people's power which is why Gamemasters must get involved.
Preventative measures have now been placed in order to discourage power abusers from abusing their power. The skull system has been adjusted to make getting red skulls easier and black skulls which prevent these players from killing anymore people unjustly. The character world transfer has also been implemented giving these oppressed players the chance to move worlds if they feel as though this is the only option that remains.
As of Summer Update 2009 extreme power abuse is no longer considered a rule violation therefore any type of in-game oppression is permitted.