Merriam-Webster's Collegiate® Dictionary, 11th Edition
LEAVEN
I. noun see: lever
Date: 14th century
1. a substance (as yeast) used to produce fermentation in dough or a liquid, a material (as baking powder) used to produce a gas that lightens dough or batter, something that modifies or lightens,
II. transitive verb (leavened; leavening)
Date: 15th century
to raise (as bread) with a leaven, to mingle or permeate with some modifying, alleviating, or vivifying element, see: infuse