Final answer:
GERD stands for Gastroesophageal Reflux Disease, a condition where stomach acid flows back into the esophagus.
Step-by-step explanation:
The correct expansion of the medical abbreviation GERD is Gastroesophageal Reflux Disease. This condition involves the backward flow of stomach acid into the esophagus, which can lead to symptoms such as heartburn and indigestion. Lifestyle changes and antacids are common forms of treatment for managing the symptoms of GERD.