Pronunciation: (lev'un),[key] —n. 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.
—v.t. 1. to add leaven to (dough or batter) and cause to rise.
2. to permeate with an altering or transforming element.