yeast

noun

any of various single-celled ascomycetous fungi in which little or no mycelium develops and which ordinarily reproduce by budding: they ferment sugars to form alcohol and carbon dioxide; esp.,
1. any of various yeasts (esp. genus Saccharomyces) used in food production, as in making beer, wine, etc. and as a leavening in baking
2. any of various yeasts (esp. genus Candida) causing infections or diseases
brewer's yeast
1. the yellowish, moist mass of yeast plants occurring as a froth on fermenting solutions
2. this substance dried in flakes or granules or compressed into cakes
foam; froth
1. something that agitates or causes ferment; leaven
2. ferment; agitation

intransitive verb

Rare to froth or ferment

See yeast in American Heritage Dictionary 4