unicellular fungi used as a leavening agent in baking and for fermenting beverages; the microorganisms (often Saccharomyces species) that cause fermentation.

パンなどの発酵や酒の醸造に使われる単細胞の菌類（酵母）

types or strains of these organisms (countable usage): particular species or varieties of yeast.

（可算）特定の酵母の種類・株

figurative: a source or cause of growth, development, or stirring activity (something that produces rapid change).

比喩的に、成長や変化を引き起こす原因・刺激（変化を生むもの）

third-person singular present form of 'yeast': to add or apply yeast to dough or a liquid to cause fermentation; to leaven or ferment.

（動詞の三人称単数現在）生地や液体に酵母を加えて発酵させる、発酵させること