yeast

American Heritage® Dictionary of the English Language, Fourth Edition

1. n. Any of various unicellular fungi of the genus Saccharomyces, especially S. cerevisiae, reproducing by budding and from ascospores and capable of fermenting carbohydrates.
2. n. Any of various similar fungi.
3. n. 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.
4. n. A powdered or compressed commercial preparation, having yeast cells and inert material such as meal and used chiefly as a leavening agent or as a dietary supplement.
5. n. Foam; froth.
6. n. An agent of ferment or activity: political agitators who are the yeast of revolution.
7. v. To ferment.
8. v. To froth or foam.

Century Dictionary and Cyclopedia

1. n. A yellowish substance, having an acid reaction, produced during the alcoholic fermentation of saccharine fluids, rising partly to the surface in the form of frothy, flocculent, viscid matter (top or surface yeast), and partly falling to the bottom (bottom or sediment yeast). Yeast consists of aggregations of minute cells, each cell constituting a distinct plant, Saccharomyces cerevisiæ. The yeast-plant is a saprophytic fungus of uncertain systematic position, being regarded by some as a degenerate ascomycete, by others as representing a distinct class. It exists under two conditions. In the first it is in the form of transparent round or oval cells, averaging .08 mm. (.003 inch) in diameter, which increase in countless numbers by budding—that is, by the formation of a small daughtercell by the side of the mother-cell, from which it sooner or later separates. The other form consists of larger cells, which, by a division of their protoplasm, form four new cells within the parent-cell. These endogenously formed cells have been likened to the ascospores of the Ascomycetes, with which, as stated above, they are frequently classed. The former notion that the yeast-plant was only the immature condition of a mold has been effectually exploded by Brefeld's elaborate researches. Fermentation takes place sooner and goes on more rapidly when yeast is added than when the fluid is merely exposed to the atmosphere, beer-yeast possessing the property of setting up fermentation in the highest degree. Surface yeast is formed at from 65° to 77° F., and its action is rapid and irregular, whereas sediment yeast is formed at from 32° to 45°, and its action is slow and quiet. Sediment yeast is reproduced by spores, and not by buds. In their chemical relations the two do not appear to differ. Yeast varies in quality according to the nature of the liquid in which it is generated, and yeastmerchants distinguish several varieties, which are employed for different purposes according to their energy and activity. Yeast is employed to induce fermentation in the manufacture of beer and ale, and of distilled spirits, and is also the agent in producing the panary fermentation, whereby bread is rendered light, porous, and spongy. Beer-yeast is employed medicinally as a stimulant in low fevers, and is of great service in cases where, from inflammatory symptoms, wine is inadmissible. See barm, Saccharomyces, fermentation.
2. n. Spumc or foam of water; froth.
3. To ferment.

Wiktionary

1. n. An often humid, yellowish froth produced by fermenting malt worts, and used to brew beer, leaven bread, and also used in certain medicines.
2. n. A type of single-celled fungus.
3. n. A compressed cake or dried granules of this substance used for mixing with flour to make bread dough rise.
4. n. figuratively A frothy foam.
5. v. To ferment.
6. v. To rise.

GNU Webster's 1913

1. 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.
2. n. Spume, or foam, of water.

WordNet 3.0

1. n. any of various single-celled fungi that reproduce asexually by budding or division
2. n. a commercial leavening agent containing yeast cells; used to raise the dough in making bread and for fermenting beer or whiskey