leavenn.[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.)v. t.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.)