A gallstone, also called a cholelith, is a stone formed within the gallbladder out of bile components. Lithiasis (stone formation) in the gallbladder is called cholelithiasis. Gallstones are formed in the gallbladder but may pass distally into other parts of the biliary tract such as the cystic duct, common bile duct, pancreatic duct or the ampulla of Vater. Rarely, in cases of severe inflammation, gallstones may erode through the gallbladder into adherent bowel potentially causing an obstruction termed gallstone ileus.