buttermilk or sour milk <ref>to sour milk: use 1 tablespoon white vinegar plus milk to equal 1 cup.</ref>

Buttermilk is a tangy, cultured dairy product derived from the fermentation of milk with lactic acid bacteria (typically Lactococcus lactis and related strains), resulting in a slightly acidic liquid with a characteristic sour flavor. Historically, buttermilk was the liquid byproduct left after churning butter from cream, but modern commercial buttermilk is produced by bacterial fermentation. Traditional buttermilk possesses a thinner consistency than the whole milk from which it originated, with a pale, opaque appearance and a subtle sour taste ranging from mildly acidic to distinctly tangy depending on fermentation duration. Sour milk, a related product, is simply milk that has been acidified through natural souring or the addition of acid (such as vinegar or lemon juice), producing similar functional qualities in baking and cooking without the bacterial cultures of true cultured buttermilk.

Buttermilk and sour milk are essential leavening agents in baking, where their acidity reacts with baking soda to produce carbon dioxide and tender, finely crumbed baked goods—essential for pancakes, biscuits, quickbreads, cakes, and muffins. Beyond baking, buttermilk features prominently in Southern and Irish cuisines, used in coatings for fried chicken, marinades for poultry and meats, and as the base for cold soups and salad dressings. The acidic nature of buttermilk also tenderizes meat and adds subtle tangy notes to sauces and gravies. In Indian cuisine, buttermilk (known as chaach or lassi when sweetened) is a cooling beverage and cooking ingredient.