Emily Dickinson Lexicon

LEAV'EN, n. [lev'n; Fr. levain, from lever, to raise, L. levo, Eng. to lift.]

1. A mass of sour dough, which, mixed with a larger quantity of dough or paste, produces fermentation in it and renders it light. During the seven days of the passover, leaven was permitted to be in the houses of the Jews. Ex. xii.
2. Any thing which makes a general change in the mass generally means something which corrupts or depraves that with which it is mixed. Beware of the leaven of the Pharisees and of the Sadducees. – Matth. xvi.