Defense

General Information
Defense/Armor is a stat for your heroes that indicates how hard it is to land a solid, damaging physical attack on them. The higher this number is, the more protection the hero possesses. Generally, however, it does little in the way of avoiding spells until Diablo III.

Diablo I/Hellfire
Defense was first seen in Diablo I/Hellfire in the form of Armor Class. It is improved by equipping armor and shields, and by upgrading one's Dexterity. It is possible for Armor Class to go into the high 200s, if not the 300s, providing an excellent defense against melee and ranged physical attackers, improved only by choosing to equip a Shield for Blocking chances.
 * Dexterity contributes to Armor Class at the rate of +1 for every 5 points invested.
 * The Barbarian from Hellfire gains an innate +1 Armor Class for every 4 levels he attains, starting at clvl. 4
 * The Monk from Hellfire gains a bonus to his Armor Class for leveling up, dependent on the type of armor he is wearing for his chest.

Diablo II/Lord of Destruction
In Diablo II, a defender's Defense Rating is compared to an attacker's Attack Rating to determine the chance of a damaging hit. All types of Armor – Body Armor, Belts, Boots, Gloves, Helms, Shields and Shrunken Heads – contribute to your character's Defense rating. Natural Defense rating also improves by 1 for every 4 points of dexterity. Defense drops to zero while running. Defense rating only works on melee and ranged attacks; it does not prevent hits from magical attacks.

NOTE: Your Strength determines what types of armor you can equip.

Diablo III

 * Avoid confusion with equippable class of Armor items

Defense returns in Diablo III, however, it is renamed to Armor, and the functions are completely different from earlier installments in the series. While means of increasing Armor are generally unchanged, rather than giving the hero an increased chance to avoid enemy attacks, it instead provides damage reduction % against those attacks.

Dexterity originally did nothing to improve that Armor rating, unless the player was a Monk with the passive skill, Seize the Initiative, or a Demon Hunter with Awareness. As of Patch 2.1, the system is changed to allow Dexterity to boost Armor (1:1) instead of.

Strength contributes to the Armor rating in the exactly same way. This means that Monks, Demon Hunters, Barbarians and Crusaders will have an innately high Armor rating just because of their Dexterity and Strength. The damage reduction % suffers from diminishing returns, and the formulae involved depend on the level of the monster attempting to attack the hero, resulting in lower damage reduction the higher the level. Armor damage reduction stacks multiplicatively with that of resistances and plays a part in forming the character's toughness.

In addition to skills and items, one may also gain up to +25% Armor (not multiplicative with other modifiers) from Paragon levels.

Unlike other stats, Armor is naturally rolling on any non-weapon, non-jewelry gear. Maximum innate Armor per item at level 70 is 759 (helms, chest armor, pants), 674 (pauldrons), 590 (gloves, boots), 506 (belts) or 421 (bracers). Shields have the highest level of protection, up to 2024 base Armor (before the Ancient Legendary bonus, which adds another 30% to base Armor).

Bonus Armor (which is an affix, not an innate property) can roll up to 397 on any piece of gear except Chest Armors, Shields, Pants, Helms and Amulets, which may roll up to 595 (before Ancient bonus, which adds 30% on top). With the exception of possessing either Dexterity or Strength, weapons may never provide a bonus towards the wielder's Armor rating.

Bottomless Potion of the Tower can add up to 20% extra Armor for 5 seconds when used.

Warnings

 * Any class should forget the idea of stacking Strength or Dexterity to gain an Armor bonus if that stat is not their primary stat. The reason for this is twofold. Firstly, they must give up their own primary stat of choice in order to gain Strength or Dexterity, since as of the pre-expansion patch, items cannot have more than one primary stat. Secondly, affixes that give a bonus to Armor may roll higher values than the Strength and Dexterity affixes, and since Armor from attributes is added at 1:1 ratio, it is more efficient to stack Bonus Armor instead.