Stun is currently a damage type only used by a few melee weapon's jump attacks.  Keep in mind that jump attack is completely different from slam damage which is the AoE effect when jump attacks impact the ground.  Unique effects of this damage type include the ability to stun enemies which involves freezing the enemy in place and giving it a stutter animation.

Innate Stun Weapons

Stun Mods

The following mods increase the stun chance of weapons, but it is unknown if they have any relation to this damage type.