A territory with one or more features present throughout and absent or unimportant elsewhere.

To geographers, a region can be either nodal or uniform, single featured or multifeatured. A nodal region is characterized by a set of places connected to another place by lines of communication or movement. The places in the set are associated with each other because they share a common focus, even though each place may be quite different from the others.

In comparison, a uniform region is a territory with one or more features present throughout and absent or unimportant elsewhere. A uniform region may represent some characterization of the total environment of an area, including both its physical and cultural features.

Our perception of the nature of a region, of the things that together shape its personality, is based on a relatively small group of criteria. In each major section of the United States, we have tried to identify the one or two underlying themes that reflect ways in which the population has interacted (within itself or with the physical environment) to create a distinctive region. The most important identifying themes for a region may vary greatly from one region to another. It is impossible to speak of the American Southwest without a focus on aridity and water erosion, of the North without its cold winters, or of the Northeast without cities and manufacturing. The key element that establishes a total uniform region, then, is not how that section compares with others on a predetermined set of variables, but how a certain set of conditions blend there.