What does "yeast" mean?

Yeast: Spume, or foam, of water. They melt thy yeast of waves, which mar Alike the Armada's pride, or spoils of Trafalgar. Byron. 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. 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.