Aggression

Aggression is a value from 0 to 100 on NPCs in Oblivion. It is used by the AI to determine when this person will enter combat. Regular townsfolk in the game typically have a setting of 5, which means they will never attack others, but will defend themselves if they are attacked. A setting of 0 is rare and it will cause the NPC never to even fight back.

Aggression is simply the trigger level, below which the NPC's disposition to another actor falls, he will then start a fight. This takes fully into account all modifiers such as faction affiliations etc. Note that Aggression is tied to Confidence and while an NPC with a low confidence will enter combat in self defense, he may immediately flee instead of actually fighting the assailant.