The sulfate salt of barium, an alkaline, divalent metal. Barium sulfate is quite insoluble in water, and is used as a radiopaque agent to diagnose gastrointestinal medical conditions. Barium sulfate is taken by mouth or given rectally. OR A procedure in which a liquid that contains barium sulfate is put through the anus into the rectum and colon. Barium sulfate is a silver-white metallic compound that helps show pictures of the colon, rectum, and anus on an x-ray.