A metropolitan statistical area (MSA) is a region formally defined by the United States Office of Management and Budget (OMB) that consists of a densely populated urban core and its surrounding less populated suburbs. To qualify as an MSA, an area must have at least one urbanized area with a population of at least 50,000. Depending on their size, MSAs may be composed of entire counties, multiple counties, or parts of counties.

The OMB defines MSAs as having substantial social and economic interconnection with their central cities. This interconnection is often evidenced by physical and economic forces. MSAs are typically composed of communities that are close enough to each other that people are likely to engage in economic exchanges, and share common social and cultural networks.

MSAs are the primary level of geographic granularity for many statistical data sources, particularly those compiled by the United States Census Bureau. Many of these data sources contain information about social, economic, and demographic trends in an MSA over time. This makes MSAs important resources for both government officials and private-sector researchers.

MSAs have become increasingly popular for researching the historical and current conditions of cities in the context of their broader metropolitan region. By studying the data in an MSA, researchers can better understand the relative strengths, weaknesses, and trends of a city's population, economy, labor market, and other features, as compared to the rest of their metropolitan region. MSAs are also useful for understanding how a city's development is shaped by its surrounding suburbs, exurbs, and other localities.

In short, MSAs are an important statistical delineation for describing cities, their economic interconnections, and their phenomena in the context of larger metropolitan regions. They provide critical data that helps researchers to understand the conditions and trends of U.S. cities more effectively.