From Vanguard Wiki
The focal point of a mob's attention. Often mixed up with Hate. Aggro is the shortened word for aggravate which is when a monster sees an adventurer and attacks relating to the hostility of a monster, the more hostile the more likely it will aggro upon you. Often times players confuse this term with hate which is defined in the Gaming Concepts as the hidden number that the monster decides to attack a player upon. The higher the number relating to Hate, the more likely to be attacked within a battle.
Eg. That Paladin tank never loses hate even with the sorcerers nuking full-time. Of course this does not reflect on what class is the better tank, because in the end it skill>gear>class
Eg. As I was running through the forest, a giant spider came and aggroed me until I ran to safety.