Lethality

Lethality is a League of Legends stat that lets your physical attacks and abilities ignore a flat amount of the enemy's armor. Unlike percentage penetration, it is most effective against squishy, low-armor targets like mages and marksmen.

A key detail is that lethality scales with your champion's level: the same item gives little penetration early but its full value by the late game. It is the signature stat of AD assassins, who stack it to burst down priority targets. Knowing whether to build lethality or flat armor penetration depends entirely on how much armor your enemies actually have.