Micropolitan statistical area
United States micropolitan statistical areas (μSA), as defined by the Office of Management and Budget (OMB), are labor market and statistical areas in the United States centered on an urban cluster (urban area) with a population of at least 10,000 but fewer than 50,000 people. The micropolitan area designation was created in 2003.
Source: Wikipedia — Micropolitan statistical area (CC BY-SA 4.0)