The Collaborative International Dictionary of English v.0.48:

Acheron \Ach"e*ron\, n. [L., fr. Gr. ?.] (Myth.)
   A river in the Nether World or infernal regions; also, the
   infernal regions themselves. By some of the English poets it
   was supposed to be a flaming lake or gulf. --Shak.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Acheron
    n 1: (Greek mythology) a river in Hades across which the souls
         of the dead were carried by Charon [syn: Acheron, River
         Acheron]