A monstrous Lovecraftian entity, usually pictured by the masses as a gigantic humanoid with an octopus for a head, who lies "dead but dreaming" in the sunken city of R'lyeh, lying in the deepest depths of the Pacific Ocean.
"It seemed to be a sort of monster, or symbol representing a monster, of a form which only a diseased fancy could conceive. If I say that my somewhat extravagant imagination yielded simultaneous pictures of an octopus, a dragon, and a human caricature, I shall not be unfaithful to the spirit of the thing. A pulpy, tentacled head surmounted a grotesque and scaly body with rudimentary wings; but it was the general outline of the whole which made it most shockingly frightful." -- Call of Cthulhu by H. P. Lovecraft
Lovecraft also tells us that Cthulhu is pronounced "...as nearly as human organs could imitate it or human letters record it - may be taken as something like Khlul'-hloo, with the first syllable pronounced gutturally and very thickly." He goes on to say "The best approximation one can make is to grunt, bark, or cough the imperfectly formed syllables Cluh-Luh with the tip of the tongue firmly affixed to the roof of the mouth. That is, if one is a human being. Directions for other entities are naturally different."