(1):
(n.) 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.
(2):
(v. t.) To imbue; to infect; to vitiate.
(3):
(v. t.) To make light by the action of leaven; to cause to ferment.
(4):
(n.) Anything which makes a general assimilating (especially a corrupting) change in the mass.