Source : Webster's Revised Unabridged Dictionary (1913)

Bottom fermentation \Bot"tom fer`men*ta"tion\
   A slow alcoholic fermentation during which the yeast cells
   collect at the bottom of the fermenting liquid. It takes
   place at a temperature of 4[deg] - 10[deg] C. (39[deg] -
   50[deg]F.). It is used in making lager beer and wines of low
   alcohol content but fine bouquet.

Source : WordNet®

bottom fermentation
     n : a slow kind of alcoholic fermentation at a temperature low
         enough that the yeast cells can sink to the bottom of the
         fermenting liquid; used in the production of lager