The Western United States (also called the American West, the Far West, and the West) is the region comprising the westernmost states of the United States. As European settlement in the U.S. expanded westward through the centuries, the meaning of the term the West changed. Before about 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.Though no consensus exists, even among experts, for the definition of the West as a region, the U.S. Census Bureau's definition of the 13 westernmost states includes the Rocky Mountains and the Great Basin to the West Coast, and the outlying states, Alaska and Hawaii.
The West contains several major biomes, including arid and semi-arid plateaus and plains, particularly in the American Southwest; forested mountains, including two major ranges, the American Sierra Nevada and Rocky Mountains; the massive coastal shoreline of the American Pacific Coast; and the rainforests of the Pacific Northwest.