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.

Spume, or foam, of water.

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.