leaven

Short definition from Wiktionary

Noun

1. 1
   Any agent used to make dough rise or to have a similar effect on baked goods.
2. 2
   Anything that induces change, especially a corrupting or vitiating change.
3. 3
   Chametz.

Verb

1. 1
   To add a leavening agent.
2. 2
   To cause to rise by fermentation.
3. 3
   To temper an action, decision, attitude, characteristic, etc.
4. 4
   To imbue; to infect; to vitiate.
5. 5
   To rise or become larger; to prove.