leaven
     n 1: a substance used to produce fermentation in dough or a
          liquid [syn: {leavening}]
     2: an influence that works subtly to lighten or modify
        something; "his sermons benefited from a leavening of
        humor" [syn: {leavening}]
     v : cause to puff up with a leaven; "unleavened bread" [syn: {raise},
          {prove}]
Source: WordNet® 2.0