Sourdough starter

A starter is not a recipe. It is a population. Flour and Water are the habitat; Wild yeast and Lactic acid bacteria are the residents.

Feeding (refreshing) the starter is a way of selecting for microbes that raise dough rather than merely souring it. The flour you feed matters: Wheat flour brings enzymes and a particular mineral and protein profile ¹.

When a portion of starter is built into a larger, timed preferment for a specific dough, bakers often call that a Levain. The two words overlap in home kitchens; the distinction is useful in a bakery.