Acid reflux is the backup of stomach acid into the esophagus, the muscular tube that connects the throat to the stomach. It may cause burning pain in the esophagus.
Acid reflux may occur because of weakness in the ring of muscle (lower esophageal sphincter, or LES) that controls the flow of food from the esophagus into the stomach. Frequent acid reflux can lead to irritation of the esophagus (esophagitis), narrowing of the esophagus, or development of sores (ulcers) in the esophagus. Long-term exposure of the esophagus to stomach acid causes the cells that line the inside of the esophagus to be replaced by cells similar to those that line the inside of the stomach (Barrett's esophagus).
WebMD Medical Reference from Healthwise