Metropolitan Areas are large population centers, as "certified" or "defined" by the U.S. Office of Management and Budget, based on U.S. Census Data. (I've been told that the "certification" is just a rubber stamp.) Smaller Metropolitan Areas are called "Micropolitan Areas."
The Census Bureau posts current and historical Lists of Metropolitan and Micropolitan Areas.
There are a number of other official terms for population centers including Metropolitan Statistical Areas (MSAs), Consolidated Metropolitan Statistical Areas (CMSAs) and Primary Metropolitan Statistical Areas (PMSAs). For more about these various terms, see About Metropolitan and Micropolitan Statistical Areas.