chymosin

[CHYM(E) + -OS(E)² + -IN.]

Enzyme in the abomasum of calves and the stomach of human infants which clots milk by precipitation of the casein. Also known as rennin. Biosynthetic chymosin is used in cheese making (vegetable rennet). See also cheese.

The milk-curdling enzyme found in the abomasal juice of preweaned calves (before pepsin formation); a preparation from the stomach of the calf is used to coagulate milk protein in the preparation of junket and thus to facilitate its digestion. Catalyzes the conversion of caseinogen from a soluble to an insoluble form (casein or curd). Called also rennin. See also rennet.

The noun has one meaning:

Meaning #1: an enzyme that occurs in gastric juice; causes milk to coagulate
  Synonym: rennin