[1] : The Collaborative International Dictionary of English v.0.48

yeast \yeast\, n. oe. yogheest, yoghest, as. gist; akin to
   d. gest, gist, g. gischt, gaumscht, ohg. jesan, jerian, to
   ferment, g. gischen, gaumschen, gaumhren, gr. ? boiled,
   zei^n to boil, skr. yas. root111.
   1913 webster
   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.
      1913 webster

   2. spume, or foam, of water.
      1913 webster

            they melt thy yeast of waves, which mar
            alike the armada's pride, or spoils of trafalgar.
                                                  --byron.
      1913 webster
      1913 webster

   yeast cake, a mealy cake impregnated with the live germs of
      the yeast plant, and used as a conveniently transportable
      substitute for yeast.

   yeast plant 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.

   yeast powder, a baling powder, -- used instead of yeast in
      leavening bread.
      1913 webster
see also:
yeast cake yeast plant saccharomyces yeast powder 

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