Armor is an attribute shared by all units. Armor is used to determine the amount of damage reduction a unit takes from basic attack (if it's Basic Attack Armor) and ability damage (if it's Ability Damage Armor) sources. Higher armor values reduce more damage. Armor is increased with each level or with certain abilities and items.
Armor Types[]
There are two types of armor: 
Basic Armor and 
Ability Armor. They function similarly, except Basic Attack Armor will only reduce damage from basic attack damage and is only affected from basic attack penetration and shred, while Ability Armor will only reduce damage from ability damage and be affected from ability penetration and shred.
Damage Reduction[]
Damage reduction values
All units, including Structures, have an inherent armor value. Any damage dealt to a unit suffers a damage reduction depending on the target's armor.
The formula to calculate a hero's damage reduction is:
- (A − AP)/(100 + (A − AP) + 10 × (HL − 1))
Where:
- DR is damage reduction
- A is armor
- AP is armor penetration
- HL is Hero Level
In simple terms, for every unit of armor you have, you gain the equivalent of 1% health in damage reduction against the corresponding type of damage (e.g. having 110 Physical Armor makes you 110% harder to kill with Physical Damage). Additionally, heroes innately gain the equivalent of 10 armor per level in damage reduction, which is exempt from Armor Penetration.
The cap on this value is 100% damage reduction, but it requires an infinite amount of armor to reach the cap, making it impossible to obtain perfect damage reduction.