Source : Webster's Revised Unabridged Dictionary (1913)

Apollyon \A*pol"ly*on\, n. [Gr. ? destroying, fr. ?, ?, to
   destroy utterly; ? from, entirely + ? to destroy.]
   The Destroyer; -- a name used --(Rev. ix. 11) for the angel
   of the bottomless pit, answering to the Hebrew Abaddon.