A substance (yeast) used to produce fermentation (dough).
To raise or lighten a batter or dough by aeration using yeast or baking powder.
Leavening is a substance, such as yeast, baking powder, or baking soda, that is added to batters, doughs, and pastries to make them rise during baking. In the days before yeast was widely available, a piece of sourdough was often used as a leavening agent, which would be kept to ferment the next batch of bread.