Armor (referred to as Armored in some card text) is a keyword effect in Bloons Card Storm that applies to Bloons and Boss Bloons. When a Bloon with Armor takes damage from any source, it is reduced by the amount of Armor it has. If a source of damage is less than or equal to a Bloon's Armor, that Bloon takes zero damage. Shields are not affected by Armor.
Some Bloons have Armor by default. Certain cards can apply Armor to other Bloons. The amount of Armor on a Bloon is indicated by a white number on an icon of a metallic plate.
None of the playable Heroes are able to gain Armor, but Dreadbloon can give itself Armor by using its Armored card.
Armor-Piercing MOAB Mauler targeting a Lead Bloon. If it attacks, it will remove all of this Bloon's Armor before dealing 65 damage.
Armor Piercing (formatted as Armor-Piercing in some card text) is a Monkey-exclusive keyword effect that causes a Monkey to strip all Armor from a target Bloon or Boss Bloon before applying damage (even if it has a Shield). Of the Monkeys that have Armor Piercing, some have additional effects associated with Armor Piercing, such as the Lead to Gold Monkey.
Splash Damage not counting for Armor Piercing