Danger Zone

The white transparent circle displayed in The Compass (also called "aggro bubble" or "aggro circle"), that surrounds your character. Any enemy (indicated by red dots) that enters the Danger Zone (more than half of that enemy's circle has to cross into the Danger Zone) immediately becomes aware of your presence and is aggravated, causing it to attack you/cast spells on you as it's AI dictates.

The aggro entry discusses this issue in more detail.