The American frontier, also known as the Old West or the Wild West, includes the geography, history, folklore, and culture in the forward wave of American expansion in mainland North America that began with European colonial settlements in the early 17th century and ended with the admission of the last few western territories as states in 1912 (except Alaska, which was not admitted into the Union until 1959). This era of massive migration and settlement was particularly encouraged by President Thomas Jefferson following the Louisiana Purchase, giving rise to the expansionist attitude known as "Manifest Destiny" and the historians' "Frontier Thesis". The legends, historical events and folklore of the American frontier have embedded themselves into United States culture so much so that the Old West, and the Western genre of media specifically, has become one of the defining tenets and prides of American national identity.

The archetypical Old West period is generally accepted by historians to have occurred between the end of the American Civil War in 1865 until the closing of the Frontier by the Census Bureau in 1890.

By 1890, settlement in the American West had reached sufficient population density that the frontier line had disappeared; in 1890 the Census Bureau released a bulletin declaring the closing of the frontier, stating: "Up to and including 1880 the country had a frontier of settlement, but at present the unsettled area has been so broken into by isolated bodies of settlement that there can hardly be said to be a frontier line. In the discussion of its extent, its westward movement, etc., it can not, therefore, any longer have a place in the census reports."

A frontier is a zone of contact at the edge of a line of settlement. Leading theorist Frederick Jackson Turner went deeper, arguing that the frontier was the scene of a defining process of American civilization: "The frontier," he asserted, "promoted the formation of a composite nationality for the American people." He theorized it was a process of development: "This perennial rebirth, this fluidity of American life, this expansion westward...furnish[es] the forces dominating American character." Turner's ideas since 1893 have inspired generations of historians (and critics) to explore multiple individual American frontiers, but the popular folk frontier concentrates 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.

Enormous popular attention was focused on the Western United States (especially the Southwest) in the second half of the 19th century and the early 20th century, from the 1850s to the 1910s. Such media typically exaggerated the romance, anarchy, and chaotic violence of the period for greater dramatic effect. This inspired the Western genre of film, along with television shows, novels, comic books, video games, children's toys and costumes.

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." Turner himself repeatedly emphasized how the availability of free land to start new farms attracted pioneering Americans: "The existence of an area offree land, its continuous recession, and the advance of American settlement westward, explain American development." Through treaties with foreign nations and native tribes, political compromise, military conquest, the 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 ideology of Manifest destiny. In his "Frontier Thesis" (1893), Turner 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.

As the American frontier passed into history, the myths of the West in fiction and film took a firm hold in the imaginations 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."