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. Since 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. The Census Bureau defines the western United States as these 13 states: Alaska, Arizona, California, Colorado, Hawaii, Idaho, Montana, Nevada, New Mexico,Oregon, Utah, Washington, and Wyoming.