[ren.net] n [ME, fr. (assumed) ME rennen to cause to coagulate, fr. OE gerennan, fr. ge- together + (assumed) OE rennan to cause to run; akin to OHG rennen to cause to run, OE rinnan to run--more at co-, run] (15c) 1 a: the contents of the stomach of an unweaned animal and esp. 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