[yeast] n [ME yest, fr. OE gist; akin to OHG jesen, gesen to ferment, Gk zein to boil] (bef. 12c) 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 (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 c (1): a minute fungus (esp. Saccharomyces cerevisiae) that is present and functionally active in yeast, usu. has little or no mycelium, and reproduces by budding (2): any of various similar fungi (esp. orders Endomycetales and Moniliales)
2. archaic: the foam or spume of waves
3: something that causes ferment or activity "were all seething with the ~ of revolt --J. F. Dobie"
[2]yeast vi (1819): ferment, froth