Webster's English Dictionary

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sludge

sludge \'sl*j\ n [prob. alter. of slush] 1: MUD, MIRE; esp : a muddy 
   deposit (as on a riverbed)  : OOZE 2: a muddy or slushy mass, deposit, or 
   sediment : as  2a: precipitated solid matter produced by water and sewage 
   treatment proces ses 2b: muddy sediment in a steam boiler  2c: a 
   precipitate or settling (as a mixture of impurities and acid) from a  
   mineral oil 3: new sea ice forming in thin detached crystals