I. ˈyēst, especially Southern & Midland ˈēst noun
Etymology: Middle English yest, from Old English gist; akin to Old High German jesen, gesen to ferment, Greek zein to boil
Date: before 12th century
1.
a. : a yellowish surface froth or sediment that occurs especially in saccharine liquids (as fruit juices) in which it promotes alcoholic fermentation, consists largely of cells of a fungus (as the saccharomyces, Saccharomyces cerevisiae ), and is used especially in the making of alcoholic liquors and as a leaven in baking
b. : a commercial product containing yeast fungi in a moist or dry medium
c.
(1) : a unicellular fungus that is present and functionally active in yeast, usually has little or no mycelium, and reproduces by budding
(2) : any of various similar fungi
2. archaic : the foam or spume of waves
3. : something that causes ferment or activity
were all seething with the yeast of revolt — J. F. Dobie
II. intransitive verb
Date: 1819
: ferment , froth