From Diablo Wiki
Aggro is an RPG term that describes who the monsters will attack first. This measurement is called "hate" or "threat" in some titles. Aggro is short for "Aggression."
- See other reference terms.
There has never been much use of the aggro measurement in the Diablo games, where monsters have generally attacked whoever is closer, whether player or minion. Other RPGs, especially those based around raiding and large group encounters, use Aggro extensively as part of the strategy of the game. Players must manage the aggro of various monsters in those titles, so the monsters will keep attacking the tanks, rather than growing more aggro towards the casters or support characters and going directly after them. This is done through tanks using skills to raise their aggro while other characters cast various debuffs or manage their line of sight to the enemy.
A quote from the wikipedia page:
A game may calculate hate generated (aggro gained) by a player on an NPC based on factors such as:
- Whether the NPC is set to attack players on sight (aggressive).
- The distance between the NPC and the player.
- The relative strength of the player compared to the NPC.
- How much damage has been dealt to the NPC by the player.
- Debuffs and status effects inflicted on the NPC by the player.
- Heals and buffs given by the player to other players or NPCs (such as pets) who are attacking the NPC.
- Status effects on the player, such as invincibility, sleep, and death, which make him or her more or less favorable to attack.
- Hate generating or reducing abilities (such as taunts or invisibility) used by the player.
- Time elapsed since the player last generated hate on the NPC.