Heartburn (acid reflux) is a medical condition caused by the backward flow of gastric acid from the stomach (which is lined with protective mucus) into the esophagus (which does not have the same protective lining). The acid burns and scars the sensitive esophageal membrane, which is the source of the burning sensation. The proximate location of the stomach-esophagus connector is near the same level as the heart, which is why the common name for this condition (incorrectly) involves the heart.
Chronic heartburn can be treated with lifestyle changes, antacids, H2-receptor blockers or proton-pump inhibitors; however, suffers are strongly urged to consult a physician for proper diagnosis as sometimes the symptoms of angina or heart attack can mimic those of heartburn. Also, chronic gastroesophageal reflux, if untreated, may cause tissue damage (known as Barrett's esophagus), which if left untreated can be a precancerous condition.