The National Mythology Exam is coming up, and I decided that I would explore words that come from Greek mythology. My first word is lethean, an adjective, which means forgetfulness. The River Lethe (lee-thee) is one of the five rivers of the Underworld. It is the river of forgetfulness and wipes the memories of spirits who drink from it. All spirits have to drink from the River Lethe in order to forget their life on Earth.