<p><strong>Noun</strong></p><ul><li>A substance, as yeast or baking powder, that causes fermentation and expansion of dough or batter.</li><li>Fermented dough reserved for producing fermentation in a new batch of dough.</li><li>An element that produces an altering or transforming influence.</li></ul><p><strong>Verb</strong></p><ul><li>(used with object)<ul><li>To add leaven to (dough or batter) and cause to rise.</li><li>To permeate with an altering or transforming element.</li></ul></li></ul>