1. single-celled yeasts that reproduce asexually by budding; used to ferment carbohydrates
2. 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 cerevisiæ is the yeast of sedimentary beer. Also called Torula.