A chronic disorder characterized by reflux of the gastric and/or duodenal contents into the distal esophagus. It is usually caused by incompetence of the lower esophageal sphincter. Symptoms include heartburn and acid indigestion. It may cause injury to the esophageal mucosa. Retrograde flow of gastric juice (GASTRIC ACID) and/or duodenal contents (BILE ACIDS; PANCREATIC JUICE) into the distal ESOPHAGUS, commonly due to incompetence of the LOWER ESOPHAGEAL SPHINCTER.