Source : Webster's Revised Unabridged Dictionary (1913)

Sludge \Sludge\, n.
   Anything resembling mud or slush; as:
   (a) A muddy or slimy deposit from sweage.
   (b) Mud from a drill hole in boring.
   (c) Muddy sediment in a steam boiler.
   (d) Settling of cottonseed oil, used in making soap, etc.
   (e) A residuum of crude paraffin-oil distillation.

Sludge \Sludge\, n. [CF. {Slush}.]
   1. Mud; mire; soft mud; slush. --Mortimer. Tennyson.

   2. Small floating pieces of ice, or masses of saturated snow.
      --Kane.

   3. (Mining) See {Slime}, 4.

   {Sludge hole}, the hand-hole, or manhole, in a steam boiler,
      by means of which sediment can be removed.

Source : WordNet®

sludge
     n 1: the precipitate produced by sewage treatment
     2: any thick messy substance [syn: {slime}, {goo}, {gook}, {guck},
         {gunk}, {muck}, {ooze}]