Armor penetration

Armor penetration (often abbreviated as AP) is the amount of armor that an attack or spell ignores when calculating damage.

For example, an Air Magic spell that grants 25% armor penetration would deal more damage to a target than a similar spell with the same numerical damage, but no penetration. If the targeted foe has 100 armor, when the spell strikes, it will treat the target as having 75 armor instead. This would cause approximately ~54% more damage to the target than otherwise.

There are two different types of armor penetration: base armor penetration and bonus armor penetration.

Base armor penetration
Base armor penetration is the non-stackable, fixed amount of penetration listed in a skill's description, e.g. this attack has 20% armor penetration. If there is more than one source, only the highest value is used.

Bonus armor penetration
Bonus armor penetration is an additional amount of armor penetration, such as "Armor penetration +20%." These do stack and add to the largest base armor penetration.
 * Sundering weapon upgrades have a 10-20% chance to give attacks an additional 20% AP.
 * Hornbows have an additional 10% AP on their attacks.
 * Judge's Insight gives an additional 20% to attacks.
 * The blessing Warrior's Might grants an additional 10% AP to attacks.