The Collaborative International Dictionary of English v.0.48:

Rennin \Ren"nin\ (r[e^]n"n[i^]n), n. (Biochem.)
   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. --[Merck
   Man. 11]
   [1913 Webster]

WordNet (r) 3.0 (2006):

rennin
    n 1: an enzyme that occurs in gastric juice; causes milk to
         coagulate [syn: rennin, chymosin]