rennin(n = noun.substance) chymosin - an enzyme that occurs in gastric juice; causes milk to coagulate;
is a kind of enzyme, coagulase

   A milk-clotting enzyme obtained from the true stomach (abomasum) of a suckling calf. Mol. wt. about 31,000. Also called chymosin, rennase, and abomasal enzyme. [1913 Webster]

rennin, n. Biochem. an enzyme secreted into the stomach of unweaned mammals causing the clotting of milk.

Etymology:
RENNET + -IN