The Collaborative International Dictionary of English v.0.48:

Underworld \Un"der*world`\, n.
   1. The lower of inferior world; the world which is under the
      heavens; the earth.
      [1913 Webster]

            That overspreads (with such a reverence)
            This underworld.                      --Daniel.
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   2. The mythological place of departed souls; Hades.
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   3. The portion of the world which is below the horizon; the
      opposite side of the world; the antipodes. [R.]
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            Fresh as the first beam glittering on a sail,
            That brings our friends up from the underworld.
                                                  --Tennyson.
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   4. The inferior part of mankind. [R.] --Atterbury.
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WordNet (r) 3.0 (2006):

underworld
    n 1: the criminal class
    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]