Region is a geographical term that is used in various ways among the different branches of geography. In general, a region is a medium-scale area of land, Earth or water, smaller than the whole areas of interest (which could be, for example, the world, a nation, a river basin, mountain range, and so on), and larger than a specific site or location. A region can be seen as a collection of smaller units (as in "the New England states") or as one part of a larger whole (as in "the New England region of the United States").

Regions are areas and or the spaces used in the study of geography. A region can be defined by physical characteristics, human characteristics and functional characteristics.As a way of describing spatial areas, the concept of regions is important and widely used among the many branches of geography, each of which can describe areas in regional terms.

For example, ecoregion is a term used in environmental geography, cultural region in cultural geography, bioregion in Biogeography, and so on. The field of geography that studies regions themselves is called Regional geography.

Regions are conceptual constructs and, thus, may vary among cultures and individuals. There are regions of the world that are called hemispheres.

Natural regions

Ecoregions

hallocalled landscape mosaics, meso-ecosystems, landtype associations, and subregions, among other terms. These in turn are grouped into larger units called variously regions, ecoregions, provinces, divisions, domains, zones, ecozones, kingdoms, and so on.

Hydrological regions

The fields of hydrology and hydrography involve the study and description of water in the environment. Surface-water hydrology focuses on streams, lakes, wetlands, and other kinds of surface water (as opposed to groundwater). Hydrology is a broad field with many topics of study, including the delineation of water-based regions.

There are many systems for defining surface water regions. A basic type of stream-based region is the drainage basin, or watershed. In some cases, drainage basins are directly linked to cultural and political regions. For example, the Hudson Bay drainage basin was defined politically as Rupert's Land, the historic territory of the Hudson's Bay Company. Boundaries between drainage basins, called water divides, are frequently used as political boundaries.

Hydrologic Units

The drainage basin concept is expanded upon in hierarchical systems of hydrologic units. In the United States, an effort is being made to delineate hydrologic units in a six level hierarchy covering the entire country and adhering to a standard called the "Federal Standard for Delineation of Hydrologic Unit Boundaries". The six nested levels of hydrologic unit regions are named, from largest to smallest, regions, subregions, basins, subbasins, watersheds, and subwatersheds. The system defines 21 hydrologic unit (HU) regions in the United States, 222 HU subregions, 352 HU basins, and 2,149 HU subbasins. The delineation of 5th level watersheds and 6th level subwatersheds is not complete, but estimates predict about 22,000 watersheds and 160,000 subwatersheds in the United States.