vt to make light by the action of leaven; to cause to ferment.
2. leaven ·noun anything which makes a general assimilating (especially a corrupting) change in the mass.
3. leaven ·vt to imbue; to infect; to vitiate.
4. 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.