Acid reflux, also known as heartburn and esophageal reflux, is medically defined as "A painful burning sensation in the esophagus just below the sternum. Heartburn is usually caused by the reflux of gastric contents into the esophagus but may be caused by gastric hyperacidity or peptic ulcer. Antacids relieve the symptoms but do not cure the heartburn. See also Hiatus Hernia." Mosby's Medical Dictionary, 3rd edition, p. 548
Under normal conditions, the sphincter muscle of the esophagus prevents acid-containing stomach contents from being regurgitated back into the esophagus but if the muscle becomes lazy or perhaps relaxed, then acid reflux or heartburn will occur.