What term describes the area west of the Mississippi River?

The term "Trans-Mississippi" is used to describe the area located west of the Mississippi River. This designation is significant in American geography and history, particularly during the period of westward expansion and settlement in the 19th century. The Trans-Mississippi region encompasses a vast area that includes various geographic features and cultural developments.

Understanding this term helps clarify discussions about the territorial growth of the United States, as many historical events, including the Louisiana Purchase and the various westward migration movements, focused on this particular area. Other terms like Great Plains and Western Frontier, while related to specific geographic or historical aspects of the west, do not encompass the broader region immediately west of the Mississippi River in its entirety. Similarly, the Southwest Territory typically refers to specific areas within what is now considered the Southwestern United States, rather than the expansive Trans-Mississippi region as a whole.