leavening means: The process of making gas from a batter or dough. Baking soda, baking powder, and yeast are all used to increase the volume and texture of baked goods by producing carbon dioxide. Leavening agents are also found in eggs. Leavening agents used to be called “lifters” in earlier times. “. (in Cooking Dictionary)

What else does leavening mean?

A substance that produces rising, fermentation or ferment. (in Merlin Dictionary)