The esophageal hiatus is an opening in the diaphragm in the upper chest of the body. The diaphragm not only acts to assist the lungs in breathing, but has openings for veins and arteries, as well as the esophagus, which is the food pipe to the stomach. As the esophagus passes through the diaphragm's esophageal hiatus on its way to the stomach, it is constricted by muscles to keep stomach fluids from escaping into the esophagus or lungs when a person breathes. When the esophageal hiatus becomes enlarged and/or loose, it can no longer perform its constrictions as well and is said to be herniated or ruptured; this is a condition called a hiatal hernia.

The diaphragm, technically called the thoracic diaphragm, is a thin muscle that stretches across the chest at the bottom of the ribs, just below the lungs. It has several openings; in addition to the hole for the esophagus, the largest are the aortic hiatus and the caval opening, through which pass the artery and vein that carry blood between the heart and lungs. The esophageal hiatus is located near the center of the diaphragm, in the area known as the right crus. In addition to the esophagus, other structures that pass through the diaphragm at this location include the vagus nerve and several blood vessels.

If the esophageal hiatus becomes enlarged and herniated, it can cause health problems. Although some hiatal hernias are simply inherited genetically, most developed over years and are more common in people over age 50. Some present symptoms such as severe heartburn or gastrointestinal reflux disease (GERD), chest infections due to stomach fluids being breathed into bronchials or lungs, and vomiting; other hernias have no symptoms. If these symptoms become severe, or a person suffers severe chest pain or is unable to have bowel movements or pass gas, medical attention must be sought immediately. This could mean there is an obstruction, or the hernia has become strangulated.

When an esophageal hiatus becomes herniated, there are two main types of rupture. The sliding hiatal hernias are over 90% of cases, and are when part of the stomach and the junction of the esophagus sometimes slides up through the hiatus opening while swallowing. At the end of the swallow, the stomach slides back through the hiatus into normal position in the abdomen. A paraesophageal hernia is when the esophageal junction remains in place; however, part of the stomach bulges up past it to lay beside the esophagus in the chest area permanently. A paraesophageal hernia is dangerous; it can become strangulated and blood flow constricted, requiring surgery to correct.