by Colin McIntosh​

In 2011 the world’s population passed seven billion, only 12 years after reaching six billion; by 2017 more than 50% are expected to be living in cities. These statistics point to the fact that cities are growing at a phenomenal rate.

Demography is the ​study of ​changes in the ​population of a ​particular ​area, or of the world, over ​time, including numbers of births and deaths, migration, etc. A relatively new science, it has contributed several of the words and expressions newly added to the Cambridge dictionary.

Many of the new city dwellers will be living in a megacity – that is, an urban area of over ten million people. The process of urbanization (the growth of city populations compared to those in the countryside) affects every continent apart from Antarctica. There are currently 35 megacities in the world, five of them in China. Cities grow when the birth rate exceeds the mortality rate (or death rate), or as a result of internal or cross-border migration.

There are several ways of referring to large cities in English. A large area with a high density of buildings is a built-up area. A metropolis is a very ​large ​city, often the most ​important ​city in a ​region or ​country. A metropolitan area (also called metro area in American English and simply metro in Indian English) can include the surrounding area:

This ​guidebook ​includes a ​map of the Phoenix metro ​area.

They own businesses ​​across the four metros in ​India.

A conurbation is a ​city ​area ​containing a ​large ​number of ​people, ​formed by ​various ​towns ​growing and ​joining together:

The Dallas–Fort Worth conurbation is commonly known as the Metroplex.

According to some statisticians, the biggest city in Europe is the Blue Banana. You may not have heard of it, but it’s the name that has been given to the conurbation that stretches from Liverpool to Venice, crossing national frontiers (and the North Sea), and taking in most of England, as well as parts of Germany and Italy. It has come about because of urban sprawl, where the boundaries of built-up areas become closer and closer together until they merge. São Paulo and Rio de Janeiro (both megacities) provide further examples of urban sprawl. In these cases, part of the phenomenon consists of unregulated development by the poor. These areas are known as favelas (from Portuguese). The word has been adopted in English, but is generally used only to talk about those in Brazil. Other ways of referring to such areas include slums or shantytowns. The term tent cities is used to refer to temporary settlements such as those set up by displaced populations, such as refugees fleeing war and persecution.

News agencies have reported that, with a shortage of hotel accommodation in Rio forecast for the Olympics, some tourists can be expected to turn to the favelas for an alternative place to stay. (Read one report here.) They will at least have the benefit of some of the best views in the city, thanks to their position perched on the mountainside. However, it may be a squeeze, with the population swelling by half a million for the Games.