Bezoar

A bezoar is an accumulated mass of undigestible matter in the stomach. Bezoars are fairly common in the stomachs of large ruminant animals, where they tend to protect the stomach from foreign objects that are accidentally swallowed. They are also common in cats, particularly hair balls. However, they are rare in humans and can cause quite serious symptoms.
In most people, undigestible material merely passes through the digestive tract and is passed in feces. However, in some circumstances the undigested food remains in the stomach and forms a large mass (on occasion, filling the entire stomach). It is more common in certain people:
* Those who compulsively swallow undigestible material such as hair or bubble gum, often due to mental illness;
* Those who are on high fiber diets, particularly with vegetables;
* Those with very low stomach acid content;
* Those who take medication with enteric coatings designed to protect the stomach.
A small bezoar can be removed through the esophagus, but large bezoars must be removed surgically.
Bezoar at Wikipedia