The Collaborative International Dictionary of English v.0.48:

Hades \Ha"des\ (h[=a]"d[=e]z), n. [Gr. "a',dhs, "A'idhs; 'a
   priv. + 'idei^n to see. Cf. Un-, Wit.]
   The nether world (according to classical mythology, the abode
   of the shades, ruled over by Hades or Pluto); the invisible
   world; the grave.
   [1913 Webster]

         And death and Hades gave up the dead which were in
         them.                                    --Rev. xx. 13
                                                  (Rev. Ver.).
   [1913 Webster]

         Neither was he left in Hades, nor did his flesh see
         corruption.                              --Acts ii. 31
                                                  (Rev. Ver.).
   [1913 Webster]

         And in Hades he lifted up his eyes, being in torments.
                                                  --Luke xvi. 23
                                                  (Rev. Ver.).
   [1913 Webster]

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):

Hades
    n 1: (Greek mythology) the god of the underworld in ancient
         mythology; brother of Zeus and husband of Persephone [syn:
         Pluto, Hades, Aides, Aidoneus]
    2: (religion) the world of the dead; "No one goes to Hades with
       all his immense wealth"-Theognis [syn: Hell, Hades,
       infernal region, netherworld, Scheol, underworld]

The Devil's Dictionary (1881-1906):

HADES, n.  The lower world; the residence of departed spirits; the
place where the dead live.
    Among the ancients the idea of Hades was not synonymous with our
Hell, many of the most respectable men of antiquity residing there in
a very comfortable kind of way.  Indeed, the Elysian Fields themselves
were a part of Hades, though they have since been removed to Paris.
When the Jacobean version of the New Testament was in process of
evolution the pious and learned men engaged in the work insisted by a
majority vote on translating the Greek word "Aides" as "Hell"; but a
conscientious minority member secretly possessed himself of the record
and struck out the objectional word wherever he could find it.  At the
next meeting, the Bishop of Salisbury, looking over the work, suddenly
sprang to his feet and said with considerable excitement:  "Gentlemen,
somebody has been razing 'Hell' here!"  Years afterward the good
prelate's death was made sweet by the reflection that he had been the
means (under Providence) of making an important, serviceable and
immortal addition to the phraseology of the English tongue.