Ability Damage

is the damage dealt through any given item or weapon when using its ability.

Increasing Base Ability Damage
The skill increases ability damage by 0.5% per level, up to 30%. Increasing base and  can also indirectly increase ability damage.

Increasing Bonus Ability Damage
Having high and  can increase the ability damage of items in the game.

The Sheep Pet can increase ability damage from 0 to 50 depending on the level. The ability goes up in terms of 0.5 per level.

The Loving reforge obtained from the Red Scarf Reforge Stone can increase ability damage by 5% when applied to chestplates.

Formula
$$\text{Ability Damage} = \text{Base Ability Damage} \times \left(1+ (\frac{Mana}{100}) \times \text{Ability Scaling}\right)$$

See table below for ability scaling

Trivia
's name might be a reference to "Ability Power" from the game League of Legends