Final answer:
Acid reflux or heartburn occurs when the digestive acid flows up from the stomach and inflames the esophagus.
Step-by-step explanation:
When the digestive acid flows up from the stomach and inflames the esophagus, it is known as acid reflux or heartburn. The digestive acid, which is usually supposed to remain in the stomach, escapes into the esophagus, causing a burning sensation. This occurs when the gastro-esophageal sphincter, a ring-like muscle at the stomach end of the esophagus, is weak or relaxes inappropriately, allowing the stomach contents to flow back into the esophagus.