Something that lightens and raises; something that modifies, eases, or animates.
Example:
The speech was on a dull subject -- "Microeconomic Theory in the 1970s" -- but its leavening of humor made the time pass quickly.
The word leavening, when used in the kitchen, usually refers to yeast or baking powder. (Unleavened bread is often hard and dense; when it is used in religious ceremonies, it may be intended as a reminder of past hardship.) Young children may provide the leavening at a family reunion, and a cheerful receptionist may be the leavening in an otherwise dull office.