The hiatus is the hole in the diaphragm which the eusophagus travels through to reach the stomach, from the thoracic to the abdominal cavity. Sometimes it is called the eusophageal hiatus.

In this picture you can see the person holding the diaphragm and the area where the eusophagus and the diaphragm merge, which is labeled the hiatus.
Diaphragm-anterior-aspect.jpg