gamer slang for Aggression; a.k.a. Hate or Threat
Having the animosity of a mob.
Aggro Range refers to the distance at which a mob will notice a player and become aggressive (i.e. attack). Generally, Elite mobs have a larger Aggro Range than common mobs, while Bosses frequently have a very small Aggro Range.
Aggro refers to the amount of threat you have on a target. The person with aggro is the person currently being attacked by the mob.
Aggro Management, in a group, is the job of the Tank. If the Tank is not strong enough DPS players, such as Rogues and Mages, will steal aggro, causing the fight to quickly descend into chaos and death (which is bad).