What it is
Rennet is a set of enzymes, chiefly one called chymosin, that makes liquid milk clot into the curds every cheese starts from. The traditional source is bracing if you have not heard it before: the enzyme is drawn from the lining of the fourth stomach, the abomasum, of young calves, and the FDA's ingredient definition names it plainly as an extract from the fourth stomachs of calves, kids or lambs. Those stomachs are a byproduct of slaughter, mostly of veal calves, which is why traditional rennet is off the table for vegetarians as much as for vegans. What complicates the verdict is that most cheese has quietly stopped using it: fermentation-produced chymosin, made by putting the calf gene into bacteria, fungi or yeast and letting them brew the identical enzyme, has been sold since 1990 and by 2017 held around 90 per cent of the world rennet market. Alongside it sit microbial rennets from moulds and much older plant coagulants from thistle, nettle and fig sap, none of which involve an animal at all.
How to tell what your cheese used
The maddening part is that the label seldom names the coagulant, and where it does the words are slippery. 'Animal rennet' or a plain 'rennet' points to the calf-stomach original, while 'microbial rennet', 'vegetable rennet' and 'non-animal rennet' all mean an animal-free coagulant, and a bare 'enzymes' could be either. Because fermentation-produced chymosin counts as much the same substance as the calf version, cheeses made with it need carry no special note, and in North America roughly nine cheeses in ten are now made this way without saying so. A 'suitable for vegetarians' mark is a genuine clue, but read it precisely: it tells you the rennet is animal-free, not that the cheese is vegan, because the cheese is still made of milk. Which is the honest heart of it for a vegan: rennet is the wrong thing to worry about, since the dairy already settles the question, and the rennet only matters when you are choosing between vegetarian cheeses.