What is gastroesophageal reflux disease? Gastroesophageal reflux disease, or GERD, occurs when there is an abnormal backflow of gastric contents into the esophagus, which then irritates the esophageal lining. Find our full video library only on Osmosis: http://osms.it/more. Join millions of current and future clinicians who learn by Osmosis, along with hundreds of universities around the world