n. & v.
n. 1 a substance added to dough to make it ferment and rise, esp. yeast, or fermenting dough reserved for the purpose. 2 a a pervasive transforming influence (cf. Matt. 13:33). b (foll. by of) a tinge or admixture of a specified quality.
v.tr. 1 ferment (dough) with leaven. 2 a permeate and transform. b (foll. by with) modify with a tempering element. the old leaven traces of the unregenerate state (cf. 1 Cor. 5:6-8).
[ ME f. OF levain f. Gallo-Roman spec. use of L levamen relief f. levare lift ]