leaven

a substance used to produce fermentation in dough or a liquid

an influence that works subtly to lighten or modify something

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.

cause to puff up with a leaven

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