Source : Webster's Revised Unabridged Dictionary (1913)

Leaven \Leav"en\, v. t. [imp. & p. p. {Leavened}; p. pr. & vb.
   n. {Leavening}.]
   1. To make light by the action of leaven; to cause to
      ferment.

            A little leaven leaveneth the whole lump. --1 Cor.
                                                  v. 6.

   2. To imbue; to infect; to vitiate.

            With these and the like deceivable doctrines, he
            leavens also his prayer.              --Milton.

Leavening \Leav"en*ing\, n.
   1. The act of making light, or causing to ferment, by means
      of leaven.

   2. That which leavens or makes light. --Bacon.

Source : WordNet®

leavening
     n 1: an influence that works subtly to lighten or modify
          something; "his sermons benefited from a leavening of
          humor" [syn: {leaven}]
     2: a substance used to produce fermentation in dough or a
        liquid [syn: {leaven}]