1. A tract leading from a hollow viscera organ to the external surface: The abdominal fistula that had to be performed on Simon was necessary as a result of an injury he had had.
2. An abnormal passage or tract leading from an abdominal organ to the external surface of the abdomen: In a colostomy, an abdominal fistula extending from the bowel to an opening on the surface of the abdomen is surgically created.

A colostomy is a medical operation in which part of the colon is removed and a hole is made in the stomach through which solid waste can leave the body.