Yeast \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.

Source: Webster's Revised Unabridged Dictionary (1913)