Kogaionon was the holy mountain of the Geto-Dacians, the place where Zalmoxis stayed in an underground cave for three years. After his disappearance into Kogaionon, he was considered dead by the Getae but after three years he resurrected and showed himself to the people, who became convinced by his holy preaching when he emerged from Kogaionon.
Strabo claims that a river with the same name flowed in the vicinity.
One modern translation of Kogaionon is "sacred mountain", which would be connected to a probable Dacian word kaga meaning "sacred", attested in two early 2nd century inscriptions from Tomis.
Kogaionon's location is still under debate, but thought to be either in the area around the Dacian capital Sarmizegetusa Regia (there is a 2291m summit there called Gugu and there are speculations that it could be the holy mountain; it may also have been Dealul Grădiştei where the ruins of the sanctuaries of Sarmizegetusa are located) or even the Ceahlău mountain, because every year on 6/7 August the shadow of the mountain forms a pyramid which is thought to have been made by the Dacians.