Leaven (?), 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]