a substance, or sometimes organism, that is used when baking to cause the item to rise
An agent, such as yeast, that produces gas to make dough rise.
An ingredient that causes dough or batter to rise, lightening its texture and increasing its volume, such as beaten eggs or egg whites, baking powder, baking soda and yeast.
A leavening agent (sometimes called just leavening or leaven) is a substance used in doughs and batters that causes a foaming action. The leavening agent reacts with moisture, heat, acidity, or other triggers to produce gas that becomes trapped as bubbles within the dough. When a dough or batter is baked, it "sets" and the holes left by the gas bubbles remain, giving breads, cakes, and other baked goods their soft, sponge-like textures.