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