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.
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            A little leaven leaveneth the whole lump. --1 Cor.
                                                  v. 6.
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   2. To imbue; to infect; to vitiate.
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            With these and the like deceivable doctrines, he
            leavens also his prayer.              --Milton.
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The Collaborative International Dictionary of English v.0.48:

Leaven \Leav"en\, n. [OE. levain, levein, F. levain, L. levamen
   alleviation, mitigation; but taken in the sense of, a
   raising, that which raises, fr. levare to raise. See Lever,
   n.]
   1. Any substance that produces, or is designed to produce,
      fermentation, as in dough or liquids; esp., a portion of
      fermenting dough, which, mixed with a larger quantity of
      dough, produces a general change in the mass, and renders
      it light; yeast; barm.
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   2. Anything which makes a general assimilating (especially a
      corrupting) change in the mass.
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            Beware of the leaven of the Pharisees, which is
            hypocrisy.                            --Luke xii. 1.
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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]