This is a list of cities in the world by nominal gross domestic product (GDP). The United Nations uses three definitions for what constitutes a city, as not all cities may be classified using the same criteria. Cities may be defined as the cities proper, by the extent of their urban area, or their metropolitan regions. The methodology of calculating GDP may differ between the studies and are widely based on projections and sometimes approximate estimations, notably for cities that are not within the OECD. Refer to sources for more information. GDP for cities that have different sources might not be comparable as the definition of a city differs between them.