Armor

Armor is the League of Legends stat that reduces the physical damage you take from basic attacks and AD-based abilities. Each point provides diminishing-free, stacking effective health, so building armor against a physical-heavy enemy team dramatically increases how long you survive.

It is the direct counter to Attack Damage and is reduced by lethality and armor penetration. Tanks and bruisers buy armor items to soak damage for their team, while squishy carries pick up small amounts when facing assassins. Reading the enemy team's damage sources to decide between armor and magic resist is a fundamental defensive skill.