01 n. Any substance that produces, or is designed to produce, fermentation, as in dough or liquids; esp., a portion of fermenting dough, which, m…
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1.
Any substance that produces, or is designed to produce, fermentation, as in dough or liquids; esp., a portion of fermenting dough, which, mixed with a larger quantity of dough, produces a general change in the mass, and renders it light; yeast; barm.
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2.
Anything which makes a general assimilating (especially a corrupting) change in the mass.“Beware of the leaven of the Pharisees, which is hypocrisy.” — Luke xii. 1.