Webster's English Dictionary

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yeast

1. yeast \'ye-st\ n [ME yest, fr. OE gist; akin to MHG jest foam, Gk zein 
   t]o boil 1a: a yellowish surface froth or sediment that occurs esp. in 
   saccharine li quids (as fruit juices) in which it promotes alcoholic 
   fermentation, consists largely of cells of a fungus (family 
   Saccharomycetaceae) and is used esp. in the making of alcoholic liquors and 
   as a leaven in baking 1b: a commercial product containing yeast plants in a 
   moist or dry medium  1c1: a minute fungus (esp. Saccharomyces cerevisiae) 
   that is present  and functionally active in yeast, usu. has little or no 
   mycelium, and reproduces by budding 1c2: any of various similar fungi (esp. 
   orders Endomycetales and Moniliales ) 2: the foam or spume of waves  3: 
   something that causes ferment or activity 
2. yeast vi : FERMENT, FROTH