Armor was a form of physical protection applied to individuals (body armor) or starships and structures (hull armor).
Klingon warriors were known to wear body armor. In 2369, Ba'el showed Worf a number of Klingon objects including armor which had been hidden by her mother Gi'ral. (TNG: "Birthright, Part II") In 2372, Kor referred to Sabak's armor as a revered artifact among Klingons. (DS9: "The Sword of Kahless")
The Kalar of Rigel VII wore armor. Captain Christopher Pike, later speaking about his experience on this planet, said to Doctor Philip Boyce that he should have sensed trouble when he saw the swords and the armor. Instead, he became trapped in a fortress and had to fight one of the Kalar warriors. (TOS: "The Cage", "The Menagerie, Part I")