yeast

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun 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.
• noun Any of numerous fungi that exhibit a one-celled growth form and reproduce by budding, including certain candidas that can cause infections in humans.
• noun 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.
• noun A powdered or compressed commercial preparation of yeast cells, used chiefly as a leavening agent or as a dietary supplement.
• noun An agent of ferment or activity.

from The Century Dictionary.

• To ferment.
• noun A yellowish substance, having an acid reaction, produced during the alcoholic fermentation of saccharine fluids, rising partly to the surface in the form of a frothy, flocculent, viscid matter (top or surface yeast), and partly falling to the bottom (bottom or sediment yeast).
• noun Spumc or foam of water; froth.

from the GNU version of the Collaborative International Dictionary of English.

• noun 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.
• noun Spume, or foam, of water.
• noun a mealy cake impregnated with the live germs of the yeast plant, and used as a conveniently transportable substitute for yeast.
• noun (Bot.) the vegetable organism, or fungus, of which beer yeast consists. The yeast plant is composed of simple cells, or granules, about one three-thousandth of an inch in diameter, often united into filaments which reproduce by budding, and under certain circumstances by the formation of spores. The name is extended to other ferments of the same genus. See Saccharomyces.
• noun a baling powder, -- used instead of yeast in leavening bread.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

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

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

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