Definition of "Styx"

A bold bad man, that dar'd to call by name / Great Gorgon, prince of darknes and dead night, / At which Cocytus quakes and Styx is put to flight.

1590, Edmund Spenser, “Book I”, in The Faerie Queene. […], London: […] [John Wolfe] for William Ponsonbie, page 13

For two-and-twenty years he [Doctor Guillotin], unguillotined, shall hear nothing but guillotine, see nothing but guillotine; then dying, shall through long centuries wander, as it were, a disconsolate ghost, on the wrong side of Styx and Lethe; his name like to outlive Cæsar’s.

1837, Thomas Carlyle, chapter IV, in The French Revolution: A History […], volume I (The Bastille), London: Chapman and Hall, book IV (States-General)