The American frontier comprises the geography, history, folklore, and cultural expression of life in the forward wave of American expansion that began with English colonial settlements in the early 17th century and ended with the admission of the last mainland territories as states in 1912. "Frontier" refers to a contrasting region at the edge of a European–American line of settlement. American historians cover multiple frontiers but the folklore is focused primarily on the conquest and settlement of Native American lands west of the Mississippi River, in what is now the Midwest, Texas, the Great Plains, the Rocky Mountains, the Southwest, and the West Coast.
In 19th- and early 20th-century media, enormous popular attention was focused on the Western United States in the second half of the 19th century, a period sometimes called the "Old West" or the "Wild West". Such media typically exaggerated the romance, anarchy, and chaotic violence of the period for greater dramatic effect. This eventually inspired the Western genre of film, which spilled over into comic books, and children's toys, games and costumes. This era of massive migration and settlement was particularly encouraged by President Thomas Jefferson following the Louisiana Purchase, giving rise to the expansionist philosophy known as "Manifest destiny".
As defined by Hine and Faragher, "frontier history tells the story of the creation and defense of communities, the use of the land, the development of markets, and the formation of states." They explain, "It is a tale of conquest, but also one of survival, persistence, and the merging of peoples and cultures that gave birth and continuing life to America." Through treaties with foreign nations and native tribes; political compromise; military conquest; establishment of law and order; the building of farms, ranches, and towns; the marking of trails and digging of mines; and the pulling in of great migrations of foreigners, the United States expanded from coast to coast, fulfilling the dreams of Manifest Destiny. Historian Frederick Jackson Turner in his "Frontier Thesis" (1893) theorized that the frontier was a process that transformed Europeans into a new people, the Americans, whose values focused on equality, democracy, and optimism, as well as individualism, self-reliance, and even violence. Thus, Turner's Frontier Thesis proclaimed the westward frontier to be the defining process of American history.
As the American frontier passed into history, the myths of the West in fiction and film took a firm hold in the imagination of Americans and foreigners alike. In David Murdoch's view, America is "exceptional" in choosing its iconic self-image: "No other nation has taken a time and place from its past and produced a construct of the imagination equal to America's creation of the West."
The frontier line was the outer line of European-American settlement. It 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). Turner favored the Census Bureau definition of the "frontier line" as a settlement density of two people per square mile. The "West" was the recently settled area near that boundary. Thus, parts of the Midwest and American South, though no longer considered "western", have a frontier heritage along with the modern western states. In the 21st century, however, the term "American West" is most often used for the area west of the Great Plains.