Clabber is a type of soured milk. It is produced by allowing unpasteurized milk to turn sour (ferment) at a specific humidity and temperature. Over time, the milk thickens or curdles into a yogurt-like consistency with a strong, sour flavor. In Joy of Cooking, "Clabber... is milk that has soured to the stage of a firm curd but not to a separation of the whey."
Prior to the now-popular use of baking powder, clabber was used as a quick leavener in baking. Due to its stability, clabbered milk has been popular in areas without access to steady refrigeration.
With the rise of almost universal pasteurization of milk and the regulation of commercial sales of raw milk, the making of clabber virtually stopped because the bacteria needed to clabber the milk are killed through the pasteurization process. Buttermilk is the commercially available pasteurized product closest to clabber. A somewhat similar food can be made from pasteurized milk by adding vinegar or lemon juice to fresh milk, which causes it to curdle.
Clabber is sometimes a middle step in cheesemaking, such as for Uzbekistan's kurt, Polish twaróg and for some cultured cheeses.