Leaven, as a noun, is that which causes transformation. It’s the source of yeast that makes your bread rise, it’s the risen bread, and it’s anything that changes the nature of something else.
Coming from the French verb levare, which means “to raise,” leaven changes everything. It makes dough go from flat to loaf, and it’s also the term to describe the risen dough before it’s baked. And leaven can be anything that causes transformation — "Winning the lottery will be the leaven that allows you to travel the world." Additionally it’s a verb: "When you bake bread, you leaven the dough to make it rise."
