Real Dictionary

leaven
verb: If a situation or activity is leavened by or with something, it is made more interesting or cheerful.

leaven in American English
a small piece of fermenting dough put aside to be used for producing fermentation in a fresh batch of dough
to make (batter or dough) rise before or during baking by means of a leavening agent
noun: a substance, as yeast or baking powder, that causes fermentation and expansion of dough or batter
transitive verb: to add leaven to (dough or batter) and cause to rise

leaven in British English
to cause fermentation in (dough or batter)
noun: any substance that produces fermentation in dough or batter, such as yeast, and causes it to rise