Hole in diaphragm
The diaphragm is the muscle that separates the chest- from the abdominal cavity.
Many structures have to go thru the diaphragm in order to enter into the abdominal cavity, including the esophagus, the aorta, and the vena cava. The hiatus is the hole in the diaphragm thru which the esophagus passes. This can enlarge, allowing the stomach to "yo-yo" up into the chest: this is called a hiatal hernia