Leaven is a substance, such as yeast, used to produce fermentation in dough to cause it to rise or lighten.
Webster’s Dictionary:
n.
1. An agent, such as yeast, that causes batter or dough to rise, especially by fermentation.
2. An element, influence, or agent that works subtly to lighten, enliven, or modify a whole.
tr.v. leav·ened, leav·en·ing, leav·ens
1. To add a rising agent to.
2. To cause to rise, especially by fermentation.
3. To pervade with a lightening, enlivening, or modifying influence.