Heartburn is a burning sensation in the chest that occurs when stomach acid flows back into the esophagus, irritating its lining. This backflow, known as acid reflux, happens when the lower esophageal sphincter—a muscular valve between the stomach and esophagus—does not close properly. The discomfort is often felt behind the breastbone and may worsen after eating, when lying down, or bending over. Additional symptoms can include a sour or bitter taste in the mouth, regurgitation of stomach contents, and throat irritation. Occasional heartburn is common, but frequent or persistent episodes may indicate gastroesophageal reflux disease (GERD), which can lead to inflammation or damage of the esophagus over time.