From Wikipedia: A leavening agent (sometimes called just leavening or leaven) is a substance used in doughs and batters that causes a foaming action intended to lighten and soften the finished product.
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.
1The LORD is my shepherd; I shall not want.
2He maketh me to lie down in green pastures: he leadeth me beside the still waters.
3He restoreth my soul: he leadeth me in the paths of righteousness for his name's sake.
4Yea, though I walk through the valley of the shadow of death, I will fear no evil: for thou art with me; thy rod and thy staff they comfort me.
5Thou preparest a table before me in the presence of mine enemies: thou anointest my head with oil; my cup runneth over.
6Surely goodness and mercy shall follow me all the days of my life: and I will dwell in the house of the LORD for ever.