The Western United States, also called the American West, the Western States, the Far West, and the West, is the region comprising the westernmost U.S. states. As American settlement in the U.S. expanded westward, the meaning of the term the West changed. Before around 1800, the crest of the Appalachian Mountains was seen as the western frontier. The frontier moved westward and eventually the lands west of the Mississippi River were considered the West.

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.