A rare type of cancer that begins in cells that line the bile ducts. A bile duct is a tube that carries fluid called bile from the liver and the gallbladder to the small intestine. Cholangiocarcinoma may be found in the bile ducts inside the liver (intrahepatic) or outside the liver (extrahepatic). Cancer that forms in the area where the right and left bile ducts meet outside the liver is called Klatskin tumor. It is the most common type of cholangiocarcinoma. Also called bile duct carcinoma.