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The Western United States, commonly referred to as the American West or simply "the West," traditionally refers to the region comprising the westernmost states of the United States. Because the U.S. expanded westward after its founding, the meaning of the West has evolved over time. Prior to about 1800, the crest of the Appalachian Mountains was seen as the western frontier. Since then, the frontier moved further west and the Mississippi River was referenced as the easternmost possible boundary of the West.
In the 21st century, the states which include the Rocky Mountains and the Great Basin to the West Coast are generally considered to comprise the American West.
Besides being a purely geographical designation, "The West" also has anthropological connotations. While this region has its own internal diversity, there is arguably an overall shared history, culture (music, cuisine), mind set or world view and closely interrelated dialects of English. As with any region of such geographically large extent and varied cultural histories, many subregions of The American West possess distinguishing and idiosyncratic qualities.
The "West" had played an important part in American history; the Old West is embedded in America's folklore.
In its most extensive definition, the western U.S. is the largest region, covering more than half the land area of the United States. It is also the most geographically diverse, incorporating geographic regions such as the Pacific Coast, the temperate rainforests of the Northwest, the Rocky Mountains, the Great Plains, most of the tall-grass prairie eastward to Western Wisconsin, Illinois, the western Ozark Plateau, the western portions of the southern forests, the Gulf Coast, and all of the desert areas located in the United States (the Mojave, Sonoran, Great Basin, and Chihuahua deserts).
The states from the Rockies westward have something of a dual nature of semiarid steppes and arid deserts in the lowlands and plateaus, and mountains and coniferous forests in the uplands and coastal regions.
The region encompasses some of the Louisiana Purchase, most of the land ceded by Britain in 1818, some of the land acquired when the Republic of Texas joined the U.S., all of the land ceded by Britain in 1846, all of the land ceded by Mexico in 1848, and all of the Gadsden Purchase.
Arizona, New Mexico, Nevada, Colorado, and Utah are typically considered to be part of the southwest, and Texas and Oklahoma are frequently considered part of the Southwest as well. Idaho, Montana, Oregon, Washington, and Wyoming can be considered part of the Northwest, and the addition of the Canadian province of British Columbia comprise the Pacific Northwest. There is also another region of both southwest and northwest states called the Mountain West, which is Arizona, New Mexico, Colorado, Utah, Nevada, Montana, Idaho, and Wyoming.
The West can be divided into the Pacific States; Alaska, California, Hawaii, Oregon, and Washington, with the term West Coast usually restricted to just California, Oregon, and Washington, and the Mountain States, always Arizona, Colorado, Idaho, Montana, Nevada, New Mexico, Utah, and Wyoming. Alaska and Hawaii, being detached from the other western states, have few similarities with them, but are usually also classified as part of the West. Western Texas in the Chihuahuan Desert is also traditionally considered part of the Western U.S, though from a climatological perspective the West might be said to begin just west of Austin, TX where annual rainfall drops off significantly from what is typically experienced in the East, with a concurrent change in plant and animal species.
Some western states are grouped into regions with eastern states. Kansas, Nebraska, South Dakota and North Dakota are often included in the Midwest, which also includes states like Iowa, Illinois and Wisconsin. Arkansas, Louisiana, Oklahoma, and Texas are also considered part of the South.
It is rare for any state east of the Mississippi River to be considered part of the modern west. Historically, however, the Northwest Territory was an important early territory of the U.S., comprising the modern states of Ohio, Indiana, Illinois, Michigan and Wisconsin, as well as the northeastern part of Minnesota. Also, American sports leagues with a "Western" conference or division often have members east of the Mississippi for various reasons such as not enough true Western teams, not strictly adhering to geographic regions, etc. For example, the NBA and NHL each have a Western Conference with a member in Tennessee.

From Wikipedia.