yeast Meaning, Definition & Usage

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

WordNet

Yeast noun
Etymology
OE. ýeest, ýest, AS. gist; akin to D. gest, gist, G. gischt, gäscht, OHG. jesan, jerian, to ferment, G. gischen, gäschen, gähren, Gr. boiled, zei^n to boil, Skr. yas.
Definitions
  1. 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. Spume, or foam, of water.
    They melt thy yeast of waves, which mar Alike the Armada's pride, or spoils of Trafalgar. Byron.
    3. 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.

Webster 1913