Source : Webster's Revised Unabridged Dictionary (1913)

Saccharomyces \Sac`cha*ro*my"ces\, n. [NL., fr. Gr. ? sugar + ?,
   ?, a fungus.] (Biol.)
   A genus of budding fungi, the various species of which have
   the power, to a greater or less extent, or splitting up sugar
   into alcohol and carbonic acid. They are the active agents in
   producing fermentation of wine, beer, etc. {Saccharomyces
   cerevisi[ae]} is the yeast of sedimentary beer. Also called
   {Torula}.

Source : WordNet®

Saccharomyces cerevisiae
     n : used as a leaven in baking and brewing [syn: {baker's yeast},
          {brewer's yeast}]