A leaven is any agent that causes batter or dough to rise and expand in volume. The word is almost synonymous with yeast, which is the most common and oldest known leavening agent, but chemical substances such as baking powder and mechanical means such as carbonation can also be used. Essentially, a leaven turns dough or batter into a foam by causing numerous bubbles of gas to form; the structure sets firm during baking. Leavened bread is therefore any bread that has a relatively soft, sponge-like texture, whereas unleavened bread such as crispbreads are flat and more dense.
The French equivalent, levain is no more precise, although it has become popular in the Anglophone world to use it to refer specifically to a natural leaven, in place of the more usual starter.
Want to find out more? Look at the chapter on ingredients in the book flour and water.
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