A river of the lower world or Hades, over which the shades of the departed were ferried by Charon, and by which the gods swore their most solemn…

Greek Mythology. A river in Hades, the water of which produced, in those who drank it, forgetfulness of the past. Hence, the ‘waters of oblivion’ or…

Something regarded as resembling the mythical river Phlegethon; a river of fire, a seething torrent or turmoil.