Pronunciation: (yēst),[key] —n. 1. any of various small, single-celled fungi of the phylum Ascomycota that reproduce by fission or budding, the daughter cells often remaining attached, and that are capable of fermenting carbohydrates into alcohol and carbon dioxide.
2. any of several yeasts of the genus Saccharomyces, used in brewing alcoholic beverages, as a leaven in baking breads, and in pharmacology as a source of vitamins and proteins. Cf. bottom yeast, brewer's yeast, top yeast. 3. spume; foam.
4. ferment; agitation.
5. something that causes ferment or agitation.