The abomasum is the fourth
stomach of
cattle (
Bos taurus) or other
ruminants. Known as the "true stomach" of ruminants, it is where
primary digestion takes place.
The abomasums of suckling calves are used as a source of rennet, a rennin-containing commercial enzyme preparation used to make cheese. Tripe is sometimes made from the abomasums of adult cattle.
From the science dictionary at http://biotech.icmb.utexas.edu/