Fistula: An abnormal passageway in the body. The fistula may go from the body surface into a blind pouch or into an internal organ or go between two internal organs. For a common example, an anal fistula has an opening in the skin near the anus that leads into a blind pouch or may connect through a tunnel with the rectal canal. For a rare example, a gastropericardial fistula is a passageway between the stomach and the pericardial sac (that surrounds the heart). There are numerous types of fistulas. They are usually designated by the organs or parts they connect (anovaginal, bronchoesophageal, etc.).