Another name for sourdough starter, or specifically a portion of starter built up with a targeted feeding for a particular bake.
Some bakers distinguish between a "mother starter" (the culture you maintain long-term) and a "levain" (a portion built from the mother specifically for a bake, sometimes with a different flour or hydration). In practice, many people use the terms interchangeably. French bakers tend to say levain; everyone else tends to say starter. Same living culture either way.
