Source : Webster's Revised Unabridged Dictionary (1913)

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.

   2. Anything which makes a general assimilating (especially a
      corrupting) change in the mass.

            Beware of the leaven of the Pharisees, which is
            hypocrisy.                            --Luke xii. 1.

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.

Source : WordNet®

leaven
     v : cause to puff up with a leaven; "unleavened bread" [syn: {raise},
          {prove}]

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}]