The Collaborative International Dictionary of English v.0.48:

Rennet \Ren"net\ (r?n"n?t), n. [F. rainette, reinette, perhaps
   fr. raine a tree frog, L. rana, because it is spotted like
   this kind of frog. Cf. Ranunculus.] (Bot.)
   A name of many different kinds of apples. Cf. Reinette.
   --Mortimer.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Rennet \Ren"net\, n. [AS. rinnan, rennan, to run, cf. gerinnan
   to curdle, coagulate. [root]11. See Run, v.]
   1. The inner, or mucous, membrane of the fourth stomach of
      the calf, or other young ruminant.
      [1913 Webster]

   2. an infusion or preparation of the calf stomach lining,
      used for coagulating milk. The active principle in this
      coagulating action is the enzyme rennin. [Written also
      runnet.]
      [1913 Webster +PJC]

   Cheese rennet. (Bot.) See under Cheese.

   Rennet ferment (Physiol. Chem.), the enzyme rennin,
      present in rennet and in variable quantity in the gastric
      juice of most animals, which has the power of curdling
      milk. The enzyme presumably acts by changing the casein of
      milk from a soluble to an insoluble form.

   Rennet stomach (Anat.), the fourth stomach, or abomasum, of
      ruminants.
      [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

rennet
    n 1: a substance that curdles milk in making cheese and junket