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