Armor was a form of physical protection applied to individuals (body armor) or starships and space stations (hull armor).
The Novans on Terra Nova used carapaces of diggers as body armor, protecting their shoulders, knees, elbows, and other parts. To start a conversation, Lieutenant Malcolm Reed asked the Novan Athan if he made this body armor by himself. (ENT: "Terra Nova")
In 2267, Captain James T. Kirk postulated that the Horta, a creature that lived deep within a planet and at home in solid rock, would have to have some form of natural armor plating in order for it to survive. (TOS: "The Devil in the Dark")
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 Phil Boyce that he should have sensed trouble when he saw the swords and the armor. Instead, he became trapped in a deserted fortress and had to fight one of the Kalar warriors. (TOS: "The Cage", "The Menagerie, Part I")
Exo-plating was the standard armor of a Borg drone. (ENT: "Regeneration"; Star Trek: First Contact)
Hirogen hunters wore body armor that also acted as an environmental suit. (VOY: "Prey")
Some 24th century Starfleet and Borg vessels were equipped with ablative hull armor. (DS9: "Past Tense, Part I", "Paradise Lost"; VOY: "Message in a Bottle", "Drone")
By the early 25th century, Starfleet possessed ablative generators which could surround a vessel in extremely resilient retractable armor. (VOY: "Endgame")