Source : Webster's Revised Unabridged Dictionary (1913)

Styx \Styx\, n. [L., fr. Gr. ?.] (Class. Myth.)
   The principal river of the lower world, which had to be
   crossed in passing to the regions of the dead.

Source : WordNet®

Styx
     n : (Greek mythology) a river in Hades across which Charon
         carried dead souls [syn: {River Styx}]