\Yeast\, n. [OE. [yogh]eest, [yogh]est, AS. gist; akin to
D. gest, gist, G. gischt, g["a]scht, OHG. jesan, jerian, to
ferment, G. gischen, g["a]schen, g["a]hren, Gr. ? boiled,
zei^n to boil, Skr. yas. [root]111.]
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.
{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.