Webster's Revised Unabridged Dictionary · 1913

Phlegethon

Phlegethon , noun

[Latin, from Greek {not transcribed}, prop. present participle of {not transcribed} to blaze.]

(Class Mythology) 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