leaven

(noun) a substance used to produce fermentation in dough or a liquid. Synonyms: leavening.

(noun) an influence that works subtly to lighten or modify something. Synonyms: leavening. Example: "His sermons benefited from a leavening of humor."

(verb) cause to puff up with a leaven. Synonyms: raise, prove. Example: "Unleavened bread."