A hiatus hernia, or hiatal hernia, is when part of the stomach squeezes up into the chest through an opening (“hiatus”) in the diaphragm.

The diaphragm is a large, thin sheet of muscle between the chest and the abdomen (tummy). It’s not exactly clear what causes hiatus hernia, but it may be the result of the diaphragm becoming weak with age, or pressure on the abdomen.