DuckCorp Dico

Found one definition

1.                 From WordNet (r) 3.0 (2006):
                   
   
   
                   
                     
   leaven
       n 1: a substance used to produce fermentation in dough or a
            liquid [syn: leaven, leavening]
       2: an influence that works subtly to lighten or modify
          something; "his sermons benefited from a leavening of humor"
          [syn: leaven, leavening]
       v 1: cause to puff up with a leaven; "unleavened bread" [syn:
            raise, leaven, prove]