noun (n)

• an influence that works subtly to lighten or modify something(noun.cognition)

  Example:
  His sermons benefited from a leavening of humor.
  

  Synonym:
  leaven
  

  source: wordnet30
• a substance used to produce fermentation in dough or a liquid(noun.substance)

  source: wordnet30
• The act of making light, or causing to ferment, by means of leaven.(noun)

  source: webster1913