Leaven is a food ingredient that causes dough to rise via fermentation. Yeast is a common leavening ingredient.
See the entry for yeast for more information about the symbolic meaning of leaven and yeast, us well as unleavened.
A lifelong student of the Bible, Peter DeHaan, PhD, wrote the 700-page website ABibleADay.com to encourage people to explore the Bible. His main blog and numerous books urge Christians to push past the status quo and reconsider how they practice their faith in every area of their lives.