yeast

n. general term for a type of unicellular fungi that multiply by budding (Medicine); leavening agent (used in bread and beer)

Yeasts are a growth form of eukaryotic microorganisms classified in the kingdom Fungi, with approximately 1,500 species described. Most reproduce asexually by budding, although a few do by binary fission. Yeasts are unicellular, although some species with yeast forms may become multicellular through the formation of a string of connected budding cells known as pseudohyphae, or true hyphae as seen in most molds. Yeast size can vary greatly depending on the species, typically measuring 3–4 µm in diameter, although some yeasts can reach over 40 µm.
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Yeast a general term including single-celled, usually rounded fungi that produce by budding; some yeasts transform to a mycelial stage under certain [more]Yeast - Community and Resources

Noun
1. a commercial leavening agent containing yeast cells; used to raise the dough in making bread and for fermenting beer or whiskey
(synonym) barm
(hypernym) leaven, leavening
(hyponym) yeast cake
2. any of various single-celled fungi that reproduce asexually by budding or division
(hypernym) fungus
(hyponym) baker's yeast, brewer's yeast, Saccharomyces cerevisiae
(member-holonym) Endomycetales, order Endomycetales

(n.)
The foam, or troth (top yeast), or the sediment (bottom yeast), of beer or other in fermentation, which contains the yeast plant or its spores, and under certain conditions produces fermentation in saccharine or farinaceous substances; a preparation used for raising dough for bread or cakes, and making it light and puffy; barm; ferment.
  

(n.)
Spume, or foam, of water.
  

(n.)
A form of fungus which grows as indvidual rounded cells, rather than in a mycelium, and reproduces by budding; esp. members of the orders Endomycetales and Moniliales. Some fungi may grow both as a yeast or as a mycelium, depending on the conditions of growth.