Gastroenterology (MeSH heading) is a branch of medicine focused on the digestive system and its disorders. The name is a combination of three Ancient Greek words gaster (gen.: gastros) (stomach), enteron (intestine), and logos (reason). In the United States, Gastroenterology is an Internal Medicine Subspecialty certified by the American Board of Internal Medicine (ABIM) and the American Osteopathic Board of Internal Medicine (AOBIM). Diseases affecting the gastrointestinal tract, which includes the organs from mouth to anus, along the alimentary canal, are the focus of this specialty. Physicians practicing in this field of medicine are called gastroenterologists. ... They have usually completed the eight years of pre-medical and medical education, the year-long internship (if this is not a part of the residency), three years of an internal medicine residency, and two to three years in the gastroenterology fellowship. Some gastroenterology trainees will complete a "fourth-year" (although this is often their 7th year of graduate medical education) in Transplant Hepatology, Advanced Endoscopy, IBD, motility or other topics. Gastroenterology is not the same as colorectal or hepatobiliary surgery, which are specialty branches of general surgery.