leaven

noun

1. a substance, as yeast or baking powder, that causes fermentation and expansion of dough or batter.
2. fermented dough reserved for producing fermentation in a new batch of dough.
3. an element that produces an altering or transforming influence.

verb (used with object)

4. to add leaven to (dough or batter) and cause to rise.
5. to permeate with an altering or transforming element.

noun Also: leavening

1. any substance that produces fermentation in dough or batter, such as yeast, and causes it to rise
2. a piece of such a substance kept to ferment a new batch of dough
3. an agency or influence that produces a gradual change

verb (tr)

4. to cause fermentation in (dough or batter)
5. to pervade, causing a gradual change, esp with some moderating or enlivening influence

n.mid-14c., from Old French levain “leaven, sourdough” (12c.), from Latin levamen “alleviation, mitigation,” but used in Vulgar Latin in its literal sense of “a means of lifting, something that raises,” from levare “to raise” (see lever). Figurative use from late 14c. v.c.1400, from leaven (n.). Related: Leavened; leavening.