What is the meaning of 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.

To add a leavening agent.

To cause to rise by fermentation.

To temper an action or decision.

To imbue; to infect; to vitiate.

To rise or become larger; to prove. (Can we add an example for this sense?)

to leave

plural of leaf

Source: wiktionary.org