ETYM Old Eng. yeest, yest, AS. gist; akin to Dutch gest, gist, German gischt, gäscht, Old High Germ. jesan, jerian, to ferment, German gischen, gäschen, gähren, Greek zein to boil, Skr. yas.
One of various single-celled fungi (especially the genus Saccharomyces) that form masses of minute circular or oval cells by budding. When placed in a sugar solution the cells multiply and convert the sugar into alcohol and carbon dioxide. Yeasts are used as fermenting agents in baking, brewing, and the making of wine and spirits. Brewer’s yeast S. cerevisiae is a rich source of vitamin B.
1. A commercial leavening agent containing yeast cells; used to raise the dough in making bread and for fermenting beer or whiskey; SYN. barm.
2. Any of various single-celled fungi that reproduce asexually by budding or division.

To ferment, froth.