Old English gist "yeast," common West Germanic (cf. Middle High German gest, German Gischt "foam, froth," Old High German jesan, German gären "to ferment"), from PIE *jes- "boil, foam, froth" (cf. Sanskrit yasyati "boils, seethes," Greek zein "to boil," Welsh ias "seething, foaming").
yeast (yēst)
n.
Any of various unicellular fungi of the genus Saccharomyces, especially S. cerevisiae, reproducing by budding and from ascospores and capable of fermenting carbohydrates.
Any of various similar fungi.
A commercial preparation in either powdered or compressed form containing yeast cells and inert material and used especially as a leavening agent or as a dietary supplement.