| Lethe (pronounced lee'thee) is the river of forgetfulness or oblivion.
It is one of the rivers that circles Hades in Greek mythology.
The other rivers are:
Spirits of the dead drank of its waters to forget the sorrows of former
life before entering the Elysian Fields (Elysium). They again
drank of the river Lethe before entering a new life.
They had much, of course, to say to each other. Anchises led Aeneas to
Lethe, the river of forgetfulness, of which the souls on their way to
live again in the world about must all drink.
"A draught of long oblivion," Anchises said. And he showed his son
those who were to be their descendants, his own and Aeneas' now waiting
by the river for their time to drink and lose the memory
of what in former lives they had done and suffered. A magnificent
company they were--the future Romans, the masters of the
world. One by one Anchises pointed them out, and told of the deeds they
would do gave his son instructions how he would best
establish his home in Italy and how he could avoid or endure all the
hardships that lay before him.
From the Aeneid
Plato also mentions the River Lethe in his Republic. Following is a
passage from the last part of
The Republic: Book X. The bold added is mine to show refrences to Lethe.
All the souls had now chosen their lives, and they went in the order of
their choice to Lachesis, who sent with them the genius whom
they had severally chosen, to be the guardian of their lives and the
fulfiller of the choice: this genius led the souls first to Clotho, and
drew them within the revolution of the spindle impelled by her
hand, thus ratifying the destiny of each; and then, when they were
fastened to this, carried them to Atropos, who spun the threads and made
them irreversible, whence without turning round they
passed beneath the throne of Necessity; and when they had all passed,
they marched on in a scorching heat to the
plain of Forgetfulness, which was a barren waste destitute
of trees and verdure; and then towards evening they encamped by
the river of Unmindfulness, whose water no vessel can hold; of this
they were all obliged to drink a certain quantity, and those who were not
saved by wisdom drank more than was necessary; and each one as he drank
forgot all things. Now after they had gone to rest, about the middle of
the night there was a thunderstorm and earthquake, and then in an instant
they were driven upwards in all manner of ways to their birth,
like stars shooting. He himself was hindered from
drinking the water. But in what manner or by what means he
returned to the body he could not say; only, in the morning, awaking
suddenly, he found himself lying on the pyre.
References in the Divine Comedy:
The English word 'lethargic' is derived from this river. In chemistry, pyrolythic has origins in lethe. | 
