Title for a being meaning roughly 'The Great One Who Is Not To Be Named' and the source of Hastur's epithet of The Unspeakable. The title derives from the Classical concept of not calling upon certain gods by name as their duties were so important to the upkeep of the world that distracting them had the potential to cause terrible disasters (later modified in the case of Hastur to imply that attracting his attention would bring disaster upon the one calling upon him).
HP Lovecraft used the name in a fragment that was incorporated into his short story The Horror from the Hills, in which it would most likely serve as an epithet for Chaugnar Faugn. Lin Carter took it to be a title of the Nameless Mist.
Lovecraft provided Robert Bloch, creator of De Vermis Mysteriis, with a bit of Latin to use as an invocation from the book: "Tibi, magnum Innominandum, signa stellarum nigrarum et bufaniformis Sadoquae sigillum"--which can be translated as "To you, the great Not-to-Be-Named, signs of the black stars, and the seal of the toad-shaped Tsathoggua".