Curd is a dairy product made by coagulating the protein from milk. Curd can be used directly, but is much more frequently produced as the first step in cheese-making. The coagulation can be achieved by adding rennet or acid, both of which cause the protein molecules to adhere to each other into solid lumps. The liquid that is left behind after this operation is called whey.
Note that in Indian English, the word "curd" is used to mean home-fermented
yogurt
; see
Curd (Indian yogurt)
.