Colectomy: An operation to remove all or part of the colon (large intestine). In a partial colectomy, the surgeon removes only part of the colon. The bowel is then reconnected or an opening of the bowel (ostomy) is created on the abdominal wall to allow the contents of the bowel to exit from the body. Colectomy may be needed for treatment of diverticulitis, benign polyps of the colon, and cancer of the colon.