Used in the Pottermore version of Forgetfulness Potion (see)
Can be bought in the Pottermore Apothecary, and can be collected in Book One, Chapter 8.
"Lethe" is the Greek goddess of forgetfulness, as well as a River in the Underworld (Hades) where souls forget about their past lives, literally the "River of Forgetfulness."
Used in the Pottermore version of Forgetfulness Potion.
"lethe" = "forgetfulness" in Greek