What does the word leaven mean?

Usage examples for leaven

  1. Here, however, within the bodies of animals and vegetables we find equivocal generation a necessity; nor do I see any way out of it except by maintaining that nothing is ever either quite dead or quite alive, but that a little leaven of the one is always left in the other. – The Note-Books of Samuel Butler by Samuel Butler
  2. The religion of Christ is a leaven of action, understanding, sacrifice, and charity. – Saint Augustin by Louis Bertrand