noun

1. 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
2. brewer's yeast
3. 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
4. foam; froth
5. 1. something that agitates or causes ferment; leaven
   2. ferment; agitation

Etymology: ME yest < OE gist, akin to Ger gischt, spray, froth & OHG jesan, to ferment < IE base *yes-, to foam, boil up > Gr zein, to boil, seethe

intransitive verb