leaven

noun

1. (countable, uncountable)Any agent used to make dough rise or to have a similar effect on baked goods.
2. (countable, figuratively, uncountable)Anything that induces change, especially a corrupting or vitiating change.

   “Take heed, beware of the leaven of the Pharisees, and of the leaven of Herod.”

   “On the far side was reflected a single red and meteoric cloud, which had treasured one last crimson ray from the sunset, or perhaps nursed within it the fiery leaven.”

   “The leaven of insincerity worked through all the fibres of his being.”

3. (Judaism, countable, uncountable)Chametz.

   “On the fourteenth day of Nisan men search for leaven by candlelight.”

verb

1. (transitive)To add a leavening agent.

   “Yeast is used to leaven bread, making it rise during baking.”

2. (transitive)To cause to rise by fermentation.
3. (figuratively, transitive)To temper an action, decision, attitude, characteristic, etc.

   “Duke. No more euasion: We haue with a leauen'd, and prepared choice Proceeded to you; therefore take your honors:[...]”

   ““A tinge of Jewish blood is not a bad thing. It leavens the”—she looked at him—“stolid stupidity of the ordinary Englishman.””

   “With fresh material, taxonomic conclusions are leavened by recognition that the material examined reflects the site it occupied; a herbarium packet gives one only a small fraction of the data desirable for sound conclusions. Herbarium material does not, indeed, allow one to extrapolate safely: what you see is what you get[…]”

4. (figuratively, transitive)To imbue; to infect; to vitiate.

   “The comedian's jokes helped leaven the atmosphere during the serious meeting.”

   “Her optimism seemed to leaven the otherwise tense environment.”

   “With these and the like deceivable doctrines, he levens also his prayer.”

   “[…] pursue virtue virtuously: leven not good actions, nor render virtues disputable. Stain not fair acts with foul intentions […]”

5. To rise or become larger; to prove.