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:

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.
      [1913 Webster]

   2. Anything which makes a general assimilating (especially a
      corrupting) change in the mass.
      [1913 Webster]

            Beware of the leaven of the Pharisees, which is
            hypocrisy.                            --Luke xii. 1.
      [1913 Webster]

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]

Moby Thesaurus II by Grady Ward, 1.0:

85 Moby Thesaurus words for "leaven":
   abate, adjust to, agent, alter, alterant, alterative, alterer,
   assuage, bespread, besprinkle, box in, breathe, brew, catalyst,
   catalytic agent, circumscribe, color, condition, crawl with,
   creep with, decoct, diffuse, diminish, dredge, dye, entincture,
   extend throughout, ferment, fermentation, fill, flavor, hedge,
   hedge about, honeycomb, imbrue, imbue, impregnate, infiltrate,
   infuse, innovationist, innovator, instill, introducer,
   leave no void, leavening, limit, mitigate, moderate, modificator,
   modifier, modify, modulate, narrow, occupy, overrun, overspread,
   overswarm, palliate, penetrate, permeate, pervade, precursor,
   qualify, raise, reduce, regulate by, restrain, restrict,
   run through, saturate, season, set conditions, set limits, soften,
   steep, suffuse, swarm with, teem with, temper, tincture, tinge,
   transformer, transfuse, transmogrifier, yeast



Easton's 1897 Bible Dictionary:

Leaven
   (1.) Heb. seor (Ex. 12:15, 19; 13:7; Lev. 2:11), the remnant of
   dough from the preceding baking which had fermented and become
   acid.

     (2.) Heb. hamets, properly "ferment." In Num. 6:3, "vinegar of
   wine" is more correctly "fermented wine." In Ex. 13:7, the
   proper rendering would be, "Unfermented things [Heb. matstsoth]
   shall be consumed during the seven days; and there shall not be
   seen with thee fermented things [hamets], and there shall not be
   seen with thee leavened mass [seor] in all thy borders." The
   chemical definition of ferment or yeast is "a substance in a
   state of putrefaction, the atoms of which are in a continual
   motion."

     The use of leaven was strictly forbidden in all offerings made
   to the Lord by fire (Lev. 2:11; 7:12; 8:2; Num. 6:15). Its
   secretly penetrating and diffusive power is referred to in 1
   Cor. 5:6. In this respect it is used to illustrate the growth of
   the kingdom of heaven both in the individual heart and in the
   world (Matt. 13:33). It is a figure also of corruptness and of
   perverseness of heart and life (Matt. 16:6, 11; Mark 8:15; 1
   Cor. 5:7, 8).