Jakarta, Lagos, and Sao Paolo, and other massive population centers are changing the way we think about cities.

At festival times, the locals don sandals and cotton indigo happi coats before heaving the neighbourhood deity through the streets on an ornate palanquin. At harvest, they gather to pound rice cakes. Even in non-festival times, there is a sense of community. Traders call out in a sing-song voice, enticing customers into their tiny shops to buy fresh fish, homemade tofu, miso or traditional sweets. Yet this is not some out-of-the-way village or coastal town. This is a fairly typical residential street in Tokyo, the world’s biggest city – a megacity, no less, with a population of some 36m people.

The character of cities – and their larger cousins the megacities – is being rapidly redefined. We can no longer look at cities primarily through a European or North American lens. The great experiment in urbanisation that was played out in the advanced economies in the 19th and 20th centuries has shifted to the developing world, increasingly to Asia.

The biggest Asian cities, from Beijing to Jakarta and from Mumbai to Manila, have an entirely different feel from Tokyo. Most are poorer, grimier and lacking in Tokyo’s stupendous public transport. But they share with the Japanese capital, and some of the great conurbations of other continents, such as Lagos or São Paulo, a role in reshaping the definition of what a city means.

“Cities are simply no longer recognisable,” says David d’Heilly, who is writing a book on Tokyo as megacity. “They used to be the populated areas around religious institutions or seats of political power. These days they’re whatever an infrastructure can support.”

Many of those rushing from the countryside – at the rate of 45m people a year in Asia alone – are drawn to megacities, usually defined as those with more than 10m people. Edward Glaeser, an economics professor at Harvard, calls them “cities on steroids”.

The idea of a megacity derives from “megalopolis”, a pejorative term coined in 1918 by Oswald Spengler, the German historian. He was describing cities that had grown too large and were edging towards decline. Jean Gottmann, a French geographer, used the term more positively in the 1950s to refer to the metropolitan corridor along America’s eastern seaboard. Now, the concept has changed again to mean massive agglomerations, mostly in the developing world.

In truth, more of the world’s population is moving to second-tier cities than to the megacities. But huge conurbations have a symbolic potency. For some, they represent a brave new world in which Chinese, Indians, Brazilians and others in the developing world are clambering from poverty. For others, the megacity is nothing less than a nightmare.

The urban shift of humanity, whose number topped 7bn in October, is inexorable. In 2008, for the first time in human history, more people were living in cities than in the countryside. By this measure, Asia, where only 40 per cent of people are urban, is behind. Much of Asia’s city-building lies ahead.

In 1975, according to National Geographic, there were just three megacities. One was New York, commercial capital of the world’s greatest economic power. Another was Mexico City, a byword for the degradation of the developing world, where people crowded into filthy slums despoiled by pollution, violence and disease. The third was Tokyo, a city that had been one of the biggest in the world, with a population of 1m, at the end of the 18th century, when it was called Edo. Tokyo’s population exploded after the war as Japan surged towards western living standards. It became a new kind of city, neither western nor poor – the New York of Asia.

Thirty-five years later, those three cities have been joined by perhaps 20 new megacities. Definitions are hazy and controversial. Tokyo’s 23 wards are home to 12m people. But the greater Tokyo conglomeration, which spills into Kawasaki and Yokohama, comprises roughly 36m souls. Population sizes should not be taken too literally. Chongqing in western China officially has 30m people partly because farmers in surrounding areas are classified as belonging to the city.