Rennin is a derived term of rennet.

As nouns the difference between rennet and rennin

is that rennet is an enzyme used as the first step in making cheese, to curdle the milk and coagulate the casein in it, derived by soaking the fourth stomach of a milk-fed calf in brine while rennin is a proteolytic enzyme, obtained from the gastric juice of the abomasum of calves, used to coagulate milk and make cheese.