The Collaborative International Dictionary of English v.0.48:

Tartarus \Tar"ta*rus\ (t[aum]r"t[.a]*r[u^]s), prop. n. [L., from
   Gr. Ta`rtaros.] (Class. Myth.)
   The infernal regions, described in the Iliad as situated as
   far below Hades as heaven is above the earth, and by later
   writers as the place of punishment for the spirits of the
   wicked. By the later poets, also, the name is often used
   synonymously with Hades, or the Lower World in general.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Tartarus
    n 1: a place where the wicked are punished after death [syn:
         Gehenna, Tartarus]