Gastroesophageal reflux disease, also known as GERD or acid reflux, is a digestive disease. While many people experience acid reflux from time to time, GERD is a constant condition. During normal digestion, the ring of muscle between the esophagus and the stomach, the lower esophageal sphincter, opens to allow food to pass into the stomach. It also closes to prevent food and acidic stomach juices from flowing back into the esophagus. Acid reflux occurs when the lower esophageal sphincter relaxes at the wrong time or has become weakened, allowing stomach acid to back up into your esophagus. This causes heartburn and irritation of the lining of your esophagus. If you have mild acid reflux at least twice weekly or moderate to severe acid reflux at least once a week, this is categorized as GERD.