How do i know exactly what is a hiatal hernia?

Hiatal hernia: Is herniation, or protrusion, of a portion of stomach (which is normally in the abdomen) into the chest through the esophageal hiatus, thus "hiatal" hernia. There are different types though and some require urgent surgery, although most are of the "sliding" type that is usually harmless. It also depends on the extent of the stomach herniation and other organ involvement.