The Collaborative International Dictionary of English v.0.48:

Demogorgon \De`mo*gor"gon\ (d[=e]"m[-o]*g[^o]r*g[o^]n or
   d[e^]m"[-o]*g[^o]r*g[o^]n), n. [First mentioned by Lutatius,
   or Lactantius Placidus, the scholiast on Statius, perh. fr.
   Gr. dai`mwn god, deity + gorgo`s fierce, terrible]
   A mysterious, terrible, and evil divinity, regarded by some
   as the author of creation, by others as a great magician who
   was supposed to command the spirits of the lower world. See
   Gorgon.
   [1913 Webster]

         Orcus and Ades, and the dreaded name
         Of Demogorgon.                           --Milton.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Demogorgon
    n 1: (Greek mythology) a mysterious and terrifying deity of the
         underworld