Source : Webster's Revised Unabridged Dictionary (1913)

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

            That overspreads (with such a reverence) This
            underworld.                           --Daniel.

   2. The mythological place of departed souls; Hades.

   3. The portion of the world which is below the horizon; the
      opposite side of the world; the antipodes. [R.]

            Fresh as the first beam glittering on a sail, That
            brings our friends up from the underworld.
                                                  --Tennyson.

   4. The inferior part of mankind. [R.] --Atterbury.

Source : WordNet®

underworld
     n 1: the criminal class
     2: (religion) the world of the dead; "he didn't want to go to
        hell when he died" [syn: {Hel}, {Hell}, {Hades}, {infernal
        region}, {netherworld}, {Scheol}]