Page "American Frontier" Paragraph 4

The frontier line -- the outer line of settlement — moved steadily westward from the 1630s to the 1880s ( with occasional movements north into Maine and Vermont, south into Florida, and east from California into Nevada ).

The " West " was always the area beyond that boundary.

Most often, however, the term " American West " is used for the area west of the Mississippi River during the 19th century. Thus, the Midwest and parts of the American South, though no longer considered " western ," have a frontier heritage along with the modern western states.