Leaven

·vt To make light by the action of leaven; to cause to ferment.

II. Leaven ·noun Anything which makes a general assimilating (especially a corrupting) change in the mass.

III. Leaven ·vt To Imbue; to Infect; to Vitiate.

IV. Leaven ·noun 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.