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.