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, known as the frontier myth, 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.