Yeast n. : 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.

Yeast n. : Spume, or foam, of water.

Yeast n. : 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.

Example:
Why the Yeast i use gives a smell after making dough?

Taxonomy
yeast, sugars, yeast species, ethanol, wine yeasts, yeast cells, fermentation, true yeasts, Saccharomyces yeasts, CO2, PMID, Microbiol, Saccharomyces boulardii, wine, cells, Brettanomyces, yeast strains, dough.

Defination from the web:
yeast species Saccharomyces cerevisiae has been used in baking and fermenting alcoholic beverages for thousands of years. ... Yeast species either require oxygen for aerobic cellular respiration (obligate aerobes), or are anaerobic but also have aerobic methods of energy ...