The American frontier, also known as the Old West, and popularly known as the Wild West, encompasses the geography, history, folklore, and culture associated with 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 contiguous western territories as states in 1912. 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 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 features of American national identity.

The Frontier Thesis, also known as Turner's Thesis or American frontierism, is the argument advanced by historian Frederick Jackson Turner in 1893 that the settlement and colonization of the rugged American frontier was decisive in forming the culture of American democracy and distinguishing it from European nations. He stressed the process of "winning a wilderness" to extend the frontier line further for U.S. colonization, and the impact this had on pioneer culture and character. Turner's text takes the ideas behind Manifest Destiny and uses them to explain how American culture came to be. The features of this unique American culture included democracy, egalitarianism, uninterest in bourgeois or high culture, and an ever-present potential for violence. "American democracy was born of no theorist's dream; it was not carried in the Susan Constant to Virginia, nor in the Mayflower to Plymouth. It came out of the American forest, and it gained new strength each time it touched a new frontier," wrote Turner.