Rennet is an enzyme that is used in cheese making to curdle milk. This reaction causes the proteins and water to break apart forming curds and whey. Rennet is traditionally derived from the stomach linings of ruminant animals such as cows, goats and lambs; but there are various forms of vegetarian rennet: microbial rennet (made from molds or other microbes), fermentation-produced chymosin (the same enzyme, made by cultured microbes instead of taken from an animal), and vegetable rennet from plants such as thistle. Wondering whether a particular cheese is vegetarian? See Is cheese vegetarian?

Related: Is cheese vegetarian? · What does DOP or PDO mean on a cheese? · What is the difference between raw milk and pasteurized milk cheese?