What term is used to describe areas within a metropolitan region that are not part of the central city?

The term that describes areas within a metropolitan region that are not part of the central city is "suburb." Suburbs are typically residential areas located on the outskirts of a city, and they are characterized by lower population density compared to urban centers. They often have their own distinct community traits, including schools, parks, and shopping areas that cater primarily to the local residents.

In contrast, while "exurbs" refer to areas that are even farther out from the city, often semi-rural or rural, they do not encompass the general suburban areas. "Urban center" denotes the vibrant, densely populated area of the city itself. "Metropolis" is a term for a large city that may encompass multiple suburbs, but it does not specifically point to areas outside the central city. Thus, "suburb" is the correct term for those distinct residential areas surrounding the central city within a metropolitan region.