Any time an internal body part pushes into an area where it doesn't belong, it's called a hernia. The hiatus is an opening in the diaphragm -- the muscular wall separating the chest cavity from the abdomen.
Normally, the esophagus (food pipe) goes through the hiatus and attaches to the stomach. In a hiatal hernia (also called hiatus hernia) the stomach bulges up into the chest through that opening.
There are two main types of hiatal hernias: sliding and paraoesophageal (next to the esophagus).
In a sliding hiatal hernia, the stomach and the section of the esophagus that joins the stomach slide up into the chest through the hiatus. This is the more common type of hernia.