The hallmark symptom of gastroesophageal reflux disease (GERD) is heartburn, which presents as a burning sensation behind the sternum. This occurs due to the reflux of gastric acid into the esophagus, irritating the mucosal lining. Heartburn is a primary indicator of GERD and is often exacerbated after meals or when lying down. Other symptoms like dysphagia or hematemesis suggest complications but are not characteristic of uncomplicated GERD. Recognizing heartburn is critical for early diagnosis and management to prevent further esophageal damage.

Reference: Harrison's Principles of Internal Medicine, 20th Edition.