A global city, also called world city or sometimes alpha city or world center, is a city which is a primary node in the global economic network. The concept comes from geography and urban studies, and the idea that globalization is created, facilitated, and enacted in strategic geographic locales according to a hierarchy of importance to the operation of the global system of finance and trade.
The most complex node is the "global city", with links binding it to other cities having a direct and tangible effect on global socio-economic affairs.  The term "global city", rather than " megacity", was popularized by sociologist Saskia Sassen in her 1991 work, The Global City: New York, London, Tokyo.  "World city", meaning a city heavily involved in global trade, appeared in the May 1886 description of Liverpool, by The Illustrated London News.  Patrick Geddes later used the term "world city" in 1915.  More recently, the term has focused on a city's financial power and high technology infrastructure, with other factors becoming less relevant.  
Global city status is considered beneficial and desirable. Competing groups have developed multiple alternative methods to classify and rank world cities and to distinguish them from non-world cities.  Although there is a consensus upon leading world cities,  the chosen criteria affect which other cities are included.  Selection criteria may be based on a yardstick value (e.g., if the producer-service sector is the largest sector then city X is a world city)  or on an imminent determination (if the producer-service sector of city X is greater than the combined producer-service sectors of N other cities then city X is a world city.) 
Cities can fall from ranking, as in the case of cities that have become less cosmopolitan and less internationally renowned in the current era.
Although criteria are variable and fluid, typical characteristics of world cities are: 
- A variety of international financial services,  notably in finance, insurance, real estate, banking, accountancy, and marketing
- Headquarters of several multinational corporations
- The existence of financial headquarters, a stock exchange, and major financial institutions
- Domination of the trade and economy of a large surrounding area
- Major manufacturing centres with port and container facilities
- Considerable decision-making power on a daily basis and at a global level
- Centres of new ideas and innovation in business, economics, culture, and politics
- Centres of media and communications for global networks
- Dominance of the national region with great international significance
- High percentage of residents employed in the services sector and information sector
- High-quality educational institutions, including renowned universities, international student attendance,  and research facilities
- Multi-functional infrastructure offering some of the best legal, medical, and entertainment facilities in the country
- High diversity in language, culture, religion, and ideologies.
Global Economic Power Index
In 2015, the second Global Economic Power Index, a meta list compiled by Richard Florida, was published by The Atlantic (distinct from a namesake list  published by the Martin Prosperity Institute), with city composite rank based on five other lists.  
Global Power City Index
The Institute for Urban Strategies at The Mori Memorial Foundation in Tokyo issued a comprehensive study of global cities in 2017. They are ranked based on six categories: economy, research & development, cultural interaction, livability, environment, and accessibility, with 70 individual indicators among them. The top ten world cities are also ranked by subjective categories including manager, researcher, artist, visitor and resident. 
- Global Power City top 10: 1. London, 2. New York City, 3. Tokyo, 4. Paris, 5. Singapore, 6. Seoul, 7. Amsterdam, 8. Berlin, 9. Hong Kong, 10. Sydney.
Jon Beaverstock, Richard G. Smith and Peter J. Taylor established the Globalization and World Cities Research Network (GaWC). A roster of world cities in the GaWC Research Bulletin 5 is ranked by their connectivity through four "advanced producer services": accountancy, advertising, banking/finance, and law.  The GaWC inventory identifies three levels of global cities and several sub-ranks,  although the authors caution that "concern for city rankings operates against the spirit of the GaWC project"  (emphasis in original).
The 2004 rankings added several new indicators while continuing to rank city economics more heavily than political or cultural factors. The 2008 roster, similar to the 1998 version, is sorted into categories of Alpha world cities (with four sub-categories), Beta world cities (three sub-categories), Gamma world cities (three sub-categories) and additional cities with High sufficiency or Sufficiency presence. The cities in the 2016 rankings are: 
Alpha level cities are linked to major economic states and regions into the world economy, and are classified into four sections, Alpha ++, Alpha +, Alpha, and Alpha − cities:
Alpha ++ cities are cities most integrated with the global economy:
Alpha + cities are highly integrated cities, filling advanced service needs:
Alpha − cities:
Beta level cities are cities that link moderate economic regions to the world economy and are classified in three sections, Beta +, Beta, and Beta − cities:
Beta + cities:
Beta − cities:
- Port Louis
- Santo Domingo
- Rio de Janeiro
- Kuwait City
- Panama City
- Guatemala City
- San José
- San Juan
- San Salvador
- St. Louis
- San Diego
Gamma level cities are cities that link smaller economic regions into the world economy, and are classified into three sections, Gamma +, Gamma, and Gamma − cities:
Gamma + cities:
Gamma − cities:
Sufficiency level cities are cities that have a sufficient degree of services so as not to be overtly dependent on world cities. This is sorted into High sufficiency cities and Sufficiency cities:
High Sufficiency cities
- Porto Alegre
- La Paz
- Johor Bahru
- San Antonio
- Salt Lake City
- Phnom Penh
- Belo Horizonte
- Port of Spain
- The Hague
- Ciudad Juarez
- Palo Alto
- San Pedro Sula
- Las Vegas
- Port Elizabeth
- Santa Cruz
- Oklahoma City
- New Orleans
- Des Moines
Global Cities Index
In 2008, the American journal Foreign Policy, in conjunction with the consulting firm A.T. Kearney and the Chicago Council on Global Affairs, published a ranking of global cities, based on consultation with Saskia Sassen, Witold Rybczynski, and others.  Foreign Policy noted that "the world’s biggest, most interconnected cities help set global agendas, weather transnational dangers, and serve as the hubs of global integration. They are the engines of growth for their countries and the gateways to the resources of their regions."  The ranking is based on 27 metrics across five dimensions: business activity, human capital, information exchange, cultural experience, and political engagement and was updated in 2010, 2012, 2014, 2015 2016 and 2017. Since 2015 it is published together with a separate index called, the Global Cities Outlook: a projection of a city’s potential based on rate of change in 13 indicators across four dimensions: personal well-being, economics, innovation, and governance. 
The Wealth Report
"The Wealth Report" (a global perspective on prime property and wealth) is made by the London-based estate agent Knight Frank LLP together with the Citi Private Bank. The report includes a "Global Cities Survey", evaluating which cities are considered the most important to the world’s HNWIs (high-net-worth individuals, having over $25 million of investable assets). For the Global Cities Survey, Citi Private Bank’s wealth advisors, and Knight Frank’s luxury property specialists were asked to name the cities that they felt were the most important to HNWIs, in regard to: "economic activity", "political power", "knowledge and influence" and "quality of life".  
Global City Competitiveness Index
In 2012, the Economist Intelligence Unit ( The Economist Group), ranked the competitiveness of global cities according to their demonstrated ability to attract capital, businesses, talent and visitors. 
- Financial centre
- List of cities by GDP
- Megalopolis (city type)
- Primate city
- Ranally city rating system
- Sassen, Saskia - The global city: strategic site/new frontier
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"Sorry, London: New York Is the World's Most Economically Powerful City". The Atlantic Monthly Group. Retrieved 16 March 2015.
Our new ranking puts the Big Apple firmly on top.
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