Breaking

Aggro from enemies can be broken through several ways:

Death

Dieing will immediately reset aggro from all enemies.

Distance

When you create sufficient distance from yourself to foes, they will break.

The logic that enemies follow when they are engaged with one target is roughly as follows, and runs regularly:

Am I settled?
  No -> Am I within 1800 units from the target?
    Yes -> Can I find a path to the target?
      Yes -> Move towards target into autoattack range
      No -> Break due to pathing
    No -> Break due to distance, unless I am a ranged attacker and in a specific part of autoattacking

By creating more than 1800 in-game units of distance from all foes in a group, aggro will be broken. Similarly, if they have to move out of 1800 units of distance in an attempt to reach you, they will break.

Shadowsteps

Shadowsteps can instantly create large distances from the mobs; do note though that when you shadowstep, foes may initially run towards your position regardless of how far you have gone (more than 5000 units stops this behaviour); it is not completely understood why this happens from the server side. This only happens for ranged attackers and casters and can be countered by running the opposite direction to them once you have finished the shadowstep, effectively following the Pathing rule below.

Pathing

By positioning yourself such that mobs have to reach 1800 in-game units of distance from yourself in order to reach you, they will break.

An example is the image below: mobs from above the gap would have to walk around the gap in terrain to reach the player, and in doing so they end up breaking towards the entrance but far enough that they are not a concern.

Note

Behaviour is different when multiple persons have aggro, known as "shared aggro". 1800 units has been measured as the distance applicable to mobs in all areas of DoA as well as other general PvE areas - however there may be exceptions, do add them here.