A French term for a culture of a naturally-occurring (wild) yeast and bacteria that can leaven and flavour bread. It is refreshed periodically by replacement of a part of the culture by new flour and water, and a portion of the refreshed culture is allowed to ferment and mature (ripen) before incorporating into the final dough.

The remainder of the culture is used to begin the next batch of dough. A levain, or "sourdough," can be perpetuated for many years.

A levain or levain bread dough is generally fermented at cool temperatures. The firmer consistency and cool temperature fermentation of a levain promotes the development of lactic rather than acidic acids. Bread leavened with a levain (Pain au Levain) has a rich, complex flavour and is generally not sour.