The Collaborative International Dictionary of English v.0.48:

Phlegethon \Phleg"e*thon\, n. [L., fr. Gr. ?, prop. p. pr. of ?
   to blaze.] (Class Myth.)
   One of the principal rivers of Hades, in the channel of which
   fire flowed instead of water.

         Fierce Phlegethon,
         Whose waves of torrent fire inflame with rage.
                                                  --Milton.
   [Webster 1913 Suppl.]