Levain, also known as leaven, is the fed starter to be used for the purpose of sourdough bread making.

It is different than sourdough starter in that when building levain, it is common to feed the starter more than its typical feeding ratio (typically 1:1:1 starter-water-flour) to ensure peak activity when proofing the dough. It is also common to add additional ingredients, like sugar, to build a special type of levain, like sweet levain, for specialized bakes. Because of this, levain builds can be use to alter the flavor and texture of a sourdough bake.

Comparison chart

Sourdough Starter	Levain
Ongoing culture, fed & discarded regularly	Built once for a single bake
Typically is made up of water, flour, and starter	Can be made up of many different ingredients
Typically fed at 1:1:1 ratio of starter-flour-water	Usually fed at a higher four and water ratio