Curd, also mosaru or dahi or Thayir or Perugu, is a traditional yogurt or fermented milk product, originating from and popular throughout the Indian subcontinent. It is usually prepared from cow's milk, and sometimes buffalo milk or goat milk. The word curd is used in Indian English to refer to homemade yogurt, while the term yogurt refers to the pasteurized commercial variety known as heat-treated fermented milk.

Yogurt is a food produced by bacterial fermentation of milk. Fermentation of sugars in the milk by these bacteria produces lactic acid, which acts on milk protein to give yogurt its texture and characteristic tart flavor. Cow's milk is most commonly used to make yogurt. Milk from water buffalo, goats, ewes, mares, camels, and yaks is also used to produce yogurt. The milk used may be homogenized or not. It may be pasteurized or raw. Each type of milk produces substantially different results.