In Greek mythology, a river of Hades that caused forgetfulness in those that drank from it (5)

(Other definitions for lethe that I've seen before include "Classical river of forgetfulness" , "Oblivion; river of the underworld" , "River whose waters caused the souls of the dead to forget completely their life on Earth" , "Ethel's mythical river of forgetfulness" , "In Greek myth, the river of forgetfulness flowing through Hades" .)