Leaven

Leaven a noun denoting a raising agent for bread or other baked goods, is also a verb meaning to use such an agent. The word is derived from the Latin levamen, something that lifts or lightens. Strictly speaking, it refers to any raising agent, for example yeast or baking powder. But it is most commonly used to mean a sourdough culture, a piece reserved from a previous batch of dough and used to ‘infect’ the current batch with natural yeasts and other micro-organisms that will both raise and flavour it.