Leaven literally means "the lightener" — from the same root as levitate and elevate, because yeast makes dough lighter by filling it with gas.
A substance, especially yeast, that causes dough to rise by producing gas during fermentation; any pervasive influence that modifies or transforms something.
From Old French levain (leaven, yeast), from Latin levāmen (alleviation, relief; that which raises), from levāre (to raise, to lift, to lighten), from levis (light in weight), from PIE *legʷh- (light, having little weight). Key roots: *legʷh- (Proto-Indo-European: "light in weight").
Leaven shares its root with levitate, lever, elevate, and even carnival (farewell to meat, with the carn- from Latin carnem, but the -val possibly influenced by levāre in the sense of putting away). The word literally means "that which makes light" — and the same concept of lightness connects the physical rising of bread to the metaphorical lightening of burdens. In biblical usage, leaven is often a metaphor for corrupting influence ("the leaven of the Pharisees"), but in