"Rennet. Rennin.
Enzyme from calves' stomachs. Used in cheese-making, rennet custard (junket), and in many coagulated dairy products. Alternatives: microbial coagulating agents, bacteria culture, lemon juice, or vegetable rennet.*"
http://www.peta.org/mc/factsheet_display.asp?ID=72
"Rennet (B): extract of calf stomach. Can also be derived from bacteria and fungus. Contains the enzyme renin which clots milk"
http://veganpeace.com/ingredients/ingredients.htm
And it's not like: "could you pass me that rennet, I'm gonna coagulate some mammary gland extract to make a whey protein shake" is something we hear in our every day lives.
By the 'whey' (bad joke, eye no), whey is also the lesser of the 2 proteins found in milk and it's a common protein supplement.
Those websites give the definitions of other common animal derived ingredients as well.