a. Any of various unicellular fungi of the genus Saccharomyces, especially S. cerevisiae, reproducing asexually by budding or sexually through the production of ascospores and capable of fermenting carbohydrates.
b. Any of numerous fungi that exhibit a one-celled growth form and reproduce by budding, including certain candidas that can cause infections in humans.
2. Froth consisting of yeast cells together with the carbon dioxide they produce in the process of fermentation, present in or added to fruit juices and other substances in the production of alcoholic beverages.
3. A powdered or compressed commercial preparation of yeast cells, used chiefly as a leavening agent or as a dietary supplement.
4. An agent of ferment or activity: political agitators who are the yeast of revolution.
[Middle English yeest, from Old English gist; see yes- in the Appendix of Indo-European roots.]
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