The Collaborative International Dictionary
Bottom fermentation
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.
Wiktionary
bottom fermentation
n. (context brewing English) A slow alcoholic fermentation process carried out at low temperatures during which the yeast sinks to the bottom of the liquid. Used in brewing lager-type beers.
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