n 1 a  :  a yellowish surface froth or sediment that occurs esp. in saccharine liquids (as fruit juices) in which it promotes alcoholic fermentation, consists largely of cells of a fungus of the family Saccharomycetaceae, and is used esp. in the making of alcoholic liquors and as a leaven in baking  b  :  a commercial product containing yeast plants in a moist or dry medium  2 a  :  a minute fungus (esp. Saccharomyces cerevisiae) that is present and functionally active in yeast, usu. has little or no mycelium, and reproduces by budding  b  :  any of various similar fungi (esp. orders Endomycetales and Moniliales) yeast*like adj 
 
 
 
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