The Collaborative International Dictionary of English v.0.48:

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.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Styx
    n 1: (Greek mythology) a river in Hades across which Charon
         carried dead souls [syn: Styx, River Styx]