Merriam-Webster Collegiate Dictionary - leaven

 I. noun  Etymology: Middle English levain, from Anglo-French levein, from Vulgar Latin *levamen, from Latin levare to raise — more at lever  Date: 14th century  1.  a. a substance (as yeast) used to produce fermentation in dough or a liquid; especially sourdough  b. a material (as baking powder) used to produce a gas that lightens dough or batter  2. something that modifies or lightens  II. transitive verb  (~ed; ~ing)  Date: 15th century  1. to raise (as bread) with a ~  2. to mingle or permeate with some modifying, alleviating, or vivifying element; especially lighten a sermon ~ed with humor  Synonyms: see infuse