Ileocecal valve: A sphincter muscle valve that separates the small intestine and the large intestine. Its critical function is to limit the reflux of colonic contents into the ileum. The ileocecal valve is distinctive because it is the only site in the gastrointestinal tract that is used for Vitamin B12 and bile acid absorption. Roughly two litres of fluid enters the colon daily through the ileocecal valve.