a substance, typically yeast, that is used in dough to make it rise

a pervasive influence that modifies something or transforms it for the better

they acted as an intellectual leaven to the warriors who dominated the city

cause (dough or bread) to rise by adding yeast or another raising agent

it only take a little bit of yeast to leaven the bread

permeate and modify or transform (something) for the better

the proceedings should be leavened by humour