Armor penetration
From ThresholdRPG Wiki
Armor penetration is a system by which you can ignore a percentage of your foe's armor value from the damage calculation of white damage. As white damage (or the damage done simply by your weapon swings) is the most consistent damage in the game, this is a very strong bonus to boost, albeit a rare one.
Armor penetration as a mechanic functions through armor resistance (resist armor) on the part of the attacker.
