A global city (also called world city or sometimes alpha city or world center) is a city generally considered to be an important node in the global economic system. The concept comes from geography and urban studies and rests on the idea that globalization can be understood as largely 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 of these entities is the "global city", whereby the linkages binding a city have a direct and tangible effect on global affairs through socio-economic means. The terminology of "global city", as opposed to megacity, was popularized by the sociologist Saskia Sassen in reference to her 1991 work, "The Global City: New York, London, Tokyo" though the term "world city" to describe cities which control a disproportionate amount of global business dates to at least May 1886, to a description of Liverpool by the Illustrated London News.Patrick Geddes also used the term "world city" later in 1915. Cities can fall from such categorization, as in the case of cities that have become less cosmopolitan and less internationally renowned in the current era, e.g., Kaliningrad, Russia; Thessaloniki, Greece; and Alexandria, Egypt.

Saskia Sassen (born in The Hague, January 5, 1949) is a Dutch-American sociologist noted for her analyses of globalization and international human migration. She currently is Robert S. Lynd Professor of Sociology at Columbia University and Centennial visiting Professor at the London School of Economics. Sassen coined the term global city. She is married to the sociologist Richard Sennett.

Sassen grew up in Buenos Aires, Argentina, where her parents Willem Sassen and Miep van der Voort moved in 1950. She also spent a part of her youth in Italy and says she was "brought up in five languages."

From 1966, Sassen spent a year each at the Université de Poitiers, France, the Università degli Studi di Roma, and the Universidad de Buenos Aires, for studies in philosophy and political science. From 1969, Sassen studied sociology and economics at the University of Notre Dame, Indiana, where she obtained M.A. and Ph.D. degrees in 1971 and 1974, respectively. In addition, she obtained a French master's degree in philosophy in Poitiers in 1974. In January 2004, Sassen received the honoris causa degree in urbanism at Delft University of Technology.