leaven

Any agent used to make dough rise or to have a similar effect on baked goods.

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.

Chametz.

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

To add a leavening agent.

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

To cause to rise by fermentation.

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.”

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.

To rise or become larger; to prove.