Rennet, the stomach contents of an unweaned animal; also, the membrane lining the stomach. The word rennet also refers to preparations that contain the enzyme rennin and that are used in curdling milk. The chief source of rennet is the fourth stomach of young calves. Rennet is used in making cheese, junket, and medicinal preparations. It is also used to coagulate casein (a milk protein) in making plastics.