Buttermilk is a dairy product that is traditionally made by mixing leftover liquid from butter churning with milk. It has a thin consistency, a pale color, and a tangy, slightly acidic flavor. The sour taste of buttermilk is due to the lactic acid present in the milk.
Buttermilk is popular in India and Southeast Asia, where it is often consumed after or during meals. In some Western countries, it is also used in baking as a leavening agent or to add tanginess to dishes. Nowadays, buttermilk can also be found in supermarkets and grocery stores as a cultured dairy product made by adding lactic acid bacteria to milk.