Leaven

A substance, typically yeast, that is added to dough to make it ferment and rise. Strictly speaking it refers to any raising agent, even baking powder or baking soda. Leaven is derived from the latin word ‘levamen’ which means something that lifts or lightens.

Currently the word leaven is most commonly used to refer to a sourdough culture, typically a piece of dough reserved from a previous batch of dough and used to ‘infect’ the current batch with natural yeasts and other micro-organisms that will both rise and flavour it.