Armor is an attribute every unit and structure with health points bears. Damage by incoming attacks is reduced by the amount of armor: every point of armor reduces the damage received by one point. Units attacking in volleys, like the Zealot and the Thor, have the target's armor applied to each of their attacks.
Units commonly have a base armor ranging from 0 to 3 and gain 1 additional armor for every armor upgrade affecting them. Every combat unit has at least one armor upgrade affecting them. Buildings commonly have an armor of 1.
Armor is commonly notated in the form x (+y), expressing the unit has x base armor, and gains y armor for every armor upgrade affecting the unit.