Aggro is short for Aggro Range or Aggressive Radius. This is the distance at which an aggressive NPC will feel threatened and attack. This distance is not fixed and will be different for different NPCs. Often times, named NPCs or elite NPCs will have larger ranges than surrounding NPCs. Scout classes usually have a larger aggro range as well. All NPCs have an aggro range regardless of their aggressiveness.
An aggressive NPC's aggro range can be seen by selecting the NPC and checking the radar map. A yellow circle appears around the NPC. The aggro range is never shown to the player by a value and must be determined by the in-game distance meter.
Non-aggressive and friendly NPCs
Non-aggressive NPCs still have an aggro range but will not become hostile when it is breached. Passive NPCs, when selected, will show an expanding white circle instead of a fixed range. Friendly NPCs will show an expanding green circle.
Most aggressive NPCs are social and will attack if they notice another NPC has been attacked within their aggro range. This is an important concept to understand when pulling mobs. Pulling the NPC with the biggest aggro radius first can lessen the chances of it noticing attacks on other nearby NPCs. In the example to the right, an attack on Mob 1 will go unnoticed by Mob 2. Mob 2 will not join the fight unless otherwise provoked. However, Mob 1 will see when Mob 2 is attacked and join the fight.