The Collaborative International Dictionary of English v.0.48:

Lethe \Le"the\ (l[=e]"th[-e] or l[=e]th), n. [See Lethal.]
   Death. [Obs.] --Shak.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Lethe \Le"the\ (l[=e]"th[-e]), n. [L., fr. Gr. lh`qh, prop.,
   forgetfulness; akin to lanqa`nesqai to forget, lanqa`nein to
   escape notice.]
   1. (Class. Myth.) A river of Hades whose waters when drunk
      caused forgetfulness of the past.
      [1913 Webster]

   2. Oblivion; a draught of oblivion; forgetfulness.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Lethe
    n 1: (Greek mythology) a river in Hades; the souls of the dead
         had to drink from it, which made them forget all they had
         done and suffered when they were alive [syn: Lethe,
         River Lethe]