Definitions of leaven:

• noun:   an influence that works subtly to lighten or modify something

  Example: "His sermons benefited from a leavening of humor"

• noun:   a substance used to produce fermentation in dough or a liquid
• verb:   cause to puff up with a leaven

  Example: "Unleavened bread"

• name:  A surname (very rare: popularity rank in the U.S.: #82185)