Colostomy: An artificial exit from the colon created to divert waste through a hole in the colon and through the wall of the abdomen. A colostomy is commonly performed by severing the colon and then attaching the end leading to the stomach to the skin, through the wall of the abdomen. At the exterior opening (stoma), a bag can be attached for waste removal. The end of the colon that leads to the rectum is closed off and becomes dormant (known as a Hartmann colostomy). There are other types of colostomy procedures. Usually a colostomy is performed because of infection, blockage, cancer, or in rare instances, severe trauma of the colon.