Merriam-Webster Collegiate Dictionary - rennet

 noun  Etymology: Middle English, from Middle English *rennen to cause to coagulate, from Old English gerennan, from ge- together + *rennan to cause to run; akin to Old High German rennen to cause to run, Old English rinnan to run — more at co-, run  Date: 15th century  1.  a. the contents of the stomach of an unweaned animal and especially a calf  b. the lining membrane of a stomach or one of its compartments (as the fourth of a ruminant) used for curdling milk; also a preparation of the stomach of animals used for this purpose  2.  a. rennin  b. a substitute for rennin