The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

            A little leaven leaveneth the whole lump. --1 Cor.
                                                  v. 6.
      [1913 Webster]

   2. To imbue; to infect; to vitiate.
      [1913 Webster]

            With these and the like deceivable doctrines, he
            leavens also his prayer.              --Milton.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Leavening \Leav"en*ing\, n.
   1. The act of making light, or causing to ferment, by means
      of leaven.
      [1913 Webster]

   2. That which leavens or makes light. --Bacon.
      [1913 Webster]

WordNet (r) 3.0 (2006):

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