Definition of: yeast

(yēst) noun
1. A substance consisting of minute cells of ascomycetous fungi (genus Saccharomyces) that clump together, forming a yellow, frothy, viscous growth which, in contact with saccharine liquids, develops or increases by germination, producing fermentation by means of enzymes, in which process alcohol and carbon dioxide are produced, as in the brewing of beer and the raising of bread.
2. Such a substance mixed with flour or meal, and sold commercially.
3. Any of a family (Saccharomycetaceae) of yeast–forming fungi.
4. Froth or spume.
5. Figuratively, mental or moral ferment: the yeast of youth.
—v.i. To foam; froth. [OE gist]