Final answer:
Acid reflux or heartburn is the term for the backward flow of stomach contents, which happens when the lower esophageal sphincter fails to close correctly.
Step-by-step explanation:
The term that describes the backward flow of stomach contents is acid reflux or heartburn. This occurs when the acidic digestive juices escape into the esophagus because the lower esophageal sphincter (also known as the gastro-esophageal sphincter in humans) does not close properly. Under normal circumstances, this muscular valve prevents stomach contents from traveling upwards; however, if it fails to close adequately, acid reflux can occur, causing discomfort and potential damage to the esophagus.