Final answer:
Acid reflux occurs when stomach acid and/or contents leak backwards into the esophagus, causing a burning sensation known as heartburn. It is a common condition that can be caused by a weak or relaxed gastro-esophageal sphincter.
Step-by-step explanation:
When stomach acid and/or contents leak backwards into the esophagus, it is called acid reflux. The esophagus is a muscular tube that carries food from the mouth to the stomach, and a ring-like muscle called the gastro-esophageal sphincter normally prevents the contents of the stomach from traveling up the esophagus. However, if this sphincter is weak or relaxes inappropriately, stomach acid can flow into the esophagus, causing a burning sensation known as heartburn.