Gastroesophageal Reflux Disease, also known by its acronym, GERD, is a condition where acid from the stomach flows upwards into the esophagus. Heartburn is the most common symptom of GERD, and the only one that almost everyone experiences at some point in their life. It is classified as a disease when it happens frequently enough to interfere with a person's normal life or to damage the esophagus.
In many cases, it is caused by excess relaxation of the sphincter that keeps the top of the stomach closed while food is not going through it.
Severe cases can damage the esophagus badly. The damage can has been linked to esophageal cancer.