leaven

An agent, such as yeast, that causes batter or dough to rise, especially by fermentation.

An element, influence, or agent that works subtly to lighten, enliven, or modify a whole.

To add a rising agent to.

To cause to rise, especially by fermentation.

To pervade with a lightening, enlivening, or modifying influence.

See leven.

To excite fermentation in; raise and make light, as dough or paste.

To imbue; work upon by some invisible or powerful influence.

To ripen; mature.

A substance that produces or is designed to produce fermentation, especially in dough; specifically, a mass of fermenting dough, which, mixed with a larger quantity of dough or paste, produces fermentation in it and renders it light.