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.
Anything which makes a general assimilating (especially a corrupting) change in the mass.
To make light by the action of leaven; to cause to ferment.
To imbue; to infect; to vitiate.
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