rennet

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun An extract made from the inner lining of the fourth stomach of a calf or other young ruminant, used in cheesemaking to curdle milk.
• noun A similar enzyme-containing substance obtained from certain other animals, plants, fungi, or bacteria.

from The Century Dictionary.

• noun The fourth stomach of a calf prepared for curdling milk; the rennet-bag.
• noun Anything used to curdle milk.
• To mix or treat with rennet.
• noun A kind of apple, said to have been introduced into England in the reign of Henry VIII. Also called renneting.

from the GNU version of the Collaborative International Dictionary of English.

• noun (Bot.) A name of many different kinds of apples. Cf. reinette.
• noun The inner, or mucous, membrane of the fourth stomach of the calf, or other young ruminant.
• noun an infusion or preparation of the calf stomach lining, used for coagulating milk. The active principle in this coagulating action is the enzyme rennin.
• noun (Bot.) See under Cheese.
• noun (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.
• noun (Anat.) the fourth stomach, or abomasum, of ruminants.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun 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.
• noun A kind of apple.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a substance that curdles milk in making cheese and junket