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.

A frontier is the political and geographical area near or beyond a boundary. A frontier can also be referred to as a "front". The term came from French in the 15th century, with the meaning "borderland"—the region of a country that fronts on another country. Unlike a border—a rigid and clear-cut form of state boundary—in the most general sense a frontier can be fuzzy or diffuse. For example, the frontier between the Eastern United States and the Old West in the 1800s was an area where European American settlements gradually thinned out and gave way to Native American settlements or uninhabited land. The frontier was not always a single continuous area, as California and various large cities were populated before the land that connected those to the East.