Sour curd, also known as sour yogurt or sour cream in some contexts, is a dairy product that has undergone fermentation. It is made by fermenting milk with certain bacteria cultures, primarily lactic acid bacteria such as Lactobacillus. During the fermentation process, these bacteria convert the lactose (milk sugar) into lactic acid, which gives the curd its characteristic tangy flavor and thick texture.

Sour curd is commonly used in cooking and baking, as well as consumed as a standalone food or added to dishes as a condiment. It is often used in recipes for sauces, marinades, dips, and desserts. In some cultures, sour curd is also used as a base for beverages or served with fruits and honey as a dessert.

Due to its fermentation process, sour curd contains probiotics, which are beneficial bacteria that can promote gut health. It is also a good source of calcium, protein, and other essential nutrients found in dairy products.