A type of preferment made with wild yeast and bacteria used to leaven a bread. Levains (or starters) are usually kept by bakers for long periods (some of them for several decades). They are periodically fed so that the microorganisms present in them continue to reproduce and maintain normal metabolism. Their main purpose is to produce acids, esters and most importantly gases for the bread to develop proper texture, taste, structure and density.

A levain is usually composed exclusively of flour and water. The yeasts and bacteria are introduced naturally by the flour and by contact with the air. This mixture, if kept at room temperature, has to be refreshed every twelve hours to keep the acidity at the desired range and to keep the microorganisms from killing themselves by over-reproducing and over-acidifying their environment.