Gastroesophageal reflux disease (GERD), which some mistakenly call gastrointestinal reflux disease, is also known as acid reflux. GERD occurs when acid or other stomach contents back up in the esophagus. The most common symptom of GERD is heartburn, which, while rarely life-threatening, can greatly reduce a patient’s quality of life by affecting daily activities, sleep and what they can eat.