Manifest Destiny is a mid-nineteenth-century American term describing the obvious intent of the US to occupy the North American continent from Atlantic to Pacific Oceans.
The term was first used by John L. O'Sullivan, the editor of United States Magazine and Democratic Review in 1845. He wrote, "The fulfillment of our manifest destiny to overspread the continent alloted by providence for the free development of our yearly multiplying millions," and "the right of our manifest destiny to ... possess the whole continent which providence has given us for the development of the great experiment of liberty and federated self-government entrusted to us."
Manifest Destiny, as understood by nineteenth-century Americans, was more than simply geographic expansionism. It was understood also as a political, economic, social, and religious expansionism. The point was not simply to have American sovereignty over the land but to also bring republican self-government and Protestantism to the Natives and Mexicans that inhabited this region. These changes, often understood as "civilization," would also bring economic opportunities.
Frederick Jackson Turner expanded the idea in his The Significance of the Frontier in American History. In it, Turner argues that the frontier defined the American civilization; the hardy frontiersman and 'the Wild West' helped create an image of America that has lasted for generations. He further argued that America's individualistic and democratic traditions are firmly rooted in the frontier experience.