Regional definitions vary from source to source. This map reflects the Western United States as defined by the U.S. Census Bureau. This region is divided into Mountain States and Pacific States subregions.

The Western United States—commonly referred to as the American West or simply The West—traditionally refers to the westernmost states of the United States. As the United States has expanded westward since its founding, the definition of the West has changed over time. In the 18th and early 19th centuries the Appalachian Mountains were considered to be the boundary. Nowadays the Mississippi River is often referenced as the easternmost possible boundary of the West. This region is divided into Mountain States and Pacific States subregions. The Census Bureau defines the western United States as these states:

• Alaska
• Arizona
• California
• Colorado
• Hawaii
• Idaho
• Montana
• Nevada
• New Mexico
• Oregon
• Utah
• Washington
• Wyoming.