An altar was a structure on which offerings or sacrifices were made for religious purposes.
An altar was built on Kelemane's planet to give offerings to Tahal. When the "Ground Shaker" arrived, the Shaman ordered an alter of equal height to Tahal's to be built, so the people could give offerings to the new god. (VOY: "Blink of an Eye")
The first draft script of ENT: "The Andorian Incident" features the Vulcan Altar of C'thia.