A world city, or a world-class city, is a city with a set of somewhat subjective traits which often include the following:
- International familiarity (or "first-name" familiarity – one would say "Paris", not "Paris, France").
- Active influence and participation in international events and world affairs (for example, New York is home to the United Nations headquarters and Brussels is home to NATO headquarters).
- A fairly large population (the center of a metropolitan area with population of at least one million, typically several million).
- A major international airport that serves as an established hub for several international airlines.
- An advanced transportation system that includes several freeways and/or a large mass transit network offering multiple modes of transportation (subway, light rail, regional rail, ferry, or bus).
- In the West, several international cultures and communities (such as a Chinatown, a Little Italy, or other immigrant communities).
- International financial institutions, corporations (especially conglomerates), and stock exchanges that have influence over the world economy.
- World-renowned cultural institutions, such as museums and universities.
- A lively cultural scene, including film festivals, premieres, a thriving music or theatre scene; a symphony orchestra, an opera company, art galleries, and street performers.
- A unique cultural air and sophistication produced by its inhabitants.
- Varied retailers and eateries, upscale boutiques and hotels, and a thriving nightlife.
- Beautiful natural setting, landmarks, and specific tourist destinations.
Several powerful and influential media outlets with an international reach are based in the world cities, such as the BBC, The New York Times, Le Monde, The Chicago Tribune, The Times, and Pravda.
Some sources feel another requirement is to have hosted the Summer Olympics; however, this requirement would exclude New York City, Boston, Madrid, and other prominent cities.
In the Western World, New York, London, and Paris have been traditionally considered the "big three" world cities – not incidentally, they also serve as symbols of global capitalism. Also, Tokyo can be added to the top of the list, due to the influence of Japan in world economic affairs. However, many people have a personal list, and any two lists are likely to differ based on cultural background, values, and experience. Inhabitants of the Pacific Rim might include Beijing, Shanghai, and Seoul, on their lists. A Muslim might include Mecca as a world city; an Arab might include Cairo or Dubai; an Australian might include Sydney or Melbourne; someone from Africa might include Johannesburg. Thus, some people may reject others' claims out of unfamiliarity, ethnocentrism, or nationalism.
In certain First World countries, the ongoing migration of manufacturing jobs to Third World countries has led to significant urban decay. Therefore, to boost urban regeneration, tourism, and revenue, the goal of building a world-class city has recently become an obsession with the governments of some mid-size cities and their constituents. For example, in the United States, this is the case with cities like Louisville, Columbus, and Indianapolis.
Most such mid-size cities would not be recognized outside of their regions as "world cities" (or even within their regions). The phenomenon of world-city building, albeit with slightly more success, has also been observed in Sydney, Frankfurt, and Toronto: each of these cities have emerged as large and influential.
GaWC Inventory of World Cities
An influential attempt to define and categorise world cities was made by the Globalization and World Cities Study Group & Network (GaWC)  (http://www.lboro.ac.uk/gawc/index.html), based primarily at Loughborough University. The roster was outlined in the GaWC Research Bulletin 5  (http://www.lboro.ac.uk/gawc/rb/rb5.html) and ranked cities based on their provision of "advanced producer services" such as accountancy, advertising, banking/finance and law. The Inventory identifies three levels of world city, termed Alpha, Beta and Gamma for their relative influence. Each level contains two or three sub-ranks. There is also a fourth level of cities that show potential to become world cities in the future. This classification is not yet authoritative, but is certainly useful as a starting point for discussion.
A. ALPHA WORLD CITIES (full service world cities)
12: London, New York
11: Chicago, Hong Kong, Paris, Tokyo
10: Frankfurt, Los Angeles, Milan, Singapore
B. BETA WORLD CITIES (major world cities)
9: San Francisco, Sydney, Toronto, Zürich
8: Brussels, Madrid, São Paulo, Seoul
7: Mexico City, Moscow
C. GAMMA WORLD CITIES (minor world cities)
6: Amsterdam, Boston, Caracas, Dallas, Düsseldorf, Geneva, Houston, Jakarta, Johannesburg, Melbourne, Osaka, Prague, Santiago, Taipei, Washington, D.C., Vancouver
5: Bangkok, Beijing, Montreal, Rome, Stockholm, Warsaw
4: Atlanta, Barcelona, Berlin, Budapest, Buenos Aires, Copenhagen, Dublin, Hamburg, Istanbul, Kuala Lumpur, Manila, Miami, Minneapolis, Munich, Shanghai
D. EVIDENCE OF WORLD CITY FORMATION
Relatively strong evidence
3: Athens, Auckland, Helsinki, Luxembourg, Lyon, Mumbai, New Delhi, Philadelphia, Rio de Janeiro, Tel Aviv, Vienna
2: Abu Dhabi, Almaty, Birmingham, Bogotá, Bratislava, Brisbane, Bucharest, Cairo, Cleveland, Cologne, Detroit, Dubai, Ho Chi Minh City, Kyiv, Lima, Lisbon, Manchester, Montevideo, Oslo, Riyadh, Rotterdam, Seattle, Stuttgart, The Hague
1: Adelaide, Antwerp, Arhus, Baltimore, Bangalore, Bologna, Brasília, Calgary, Cape Town, Colombo, Columbus, Dresden, Edinburgh, Genoa, Glasgow, Gothenburg, Guangzhou, Hanoi, Kansas City, Leeds, Lille, Marseille, Richmond, Saint Petersburg, Tashkent, Teheran, Tijuana, Turin, Utrecht, Wellington, Winnipeg, Nottingham
There is also a schematic map of the GaWC cities at their website,  (http://www.lboro.ac.uk/gawc/citymap.html), which shows clearly that the great majority of their defined cities lie in the Northern Hemisphere.