Definition: [noun] Aggravation; bother.
Definition: [noun] Aggressive behaviour; loud, intimidating behaviour that convincingly threatens violence without necessarily actually becoming violent.
Example: Move along, lads; we don't want any aggro.
Definition: [noun] A measure of how belligerent a player is – a high value may inspire either avoidance or preemptive hostile action from enemies.
Definition: [noun] (MMORPG slang) Hostile attention from an enemy that should target players with better defenses.
Definition: [verb] (MMORPG slang) To become aggressive towards the player's character.
Example: Do they aggro on sight or sound?
Definition: [adjective] Angry.
Definition: [adjective] (MMORPG slang) Liable to attack without being attacked first (said of monsters).
Definition: [adjective] Hardcore, aggressive.