leaven >> ˈlɛv(ə)n
Definition: [noun] a substance, typically yeast, that is used in dough to make it rise.
Definition: [noun] a pervasive influence that modifies something or transforms it for the better.
Example: they acted as an intellectual leaven to the warriors who dominated the city
Definition: [verb] cause (dough or bread) to rise by adding yeast or another raising agent.
Example: it only take a little bit of yeast to leaven the bread
Definition: [verb] permeate and modify or transform (something) for the better.
Example: the proceedings should be leavened by humour