Generally alludes to a procedure of flavoring, or including a little proportion of something to an undifferentiated mass of another thing to improve it. Leaven is yeast which is a living being and spreads through the mixture, delivering gas which makes the bread rise. It was particularly evacuated during the Passover blowout to symbolize filtration. It is utilized figuratively as a picture of a negative behavior pattern which spreads and develops if unchecked. “A little leaven leaveneth the whole lump” Galatians 5:9 KJV