Trans-Appalachia is an area in the United States bounded to the east by the Appalachian Mountains and extending west roughly to the Mississippi River. It spans from the Midwest to the Upper South. The term is used most frequently when referring to the area as a frontier in the 17th, 18th and 19th century. During this period, the region gained its own identity, defined by its isolation and separation from the rest of the United States to the east. It included much of Ohio Country and at least the northern and eastern parts of the Old Southwest. It was never an organized territory or other political unit. Most of what was referred to by this name became the states of western Pennsylvania, Ohio, West Virginia, Kentucky, Tennessee and western Virginia. It is still a vague and little used place name today.