There are hundreds of definitions of what is a city.
"An inhabited place of greater size, population, or importance than a town or village" says Merriam-Webster, comparing it to another settlement for scale.
"City, relatively permanent and highly organized centre of population, of greater size or importance than a town or village. The name city is given to certain urban communities by virtue of some legal or conventional distinction that can vary between regions or nations. In most cases, however, the concept of city refers to a particular type of community, the urban community, and its culture, known as “urbanism.”" says Brittanica, perhaps touching upon two of the most important phrases - "distinction", i.e. boundaries, and "community", i.e. its people.
There is no 'urban' without a city. A city is 'urban'. City life = urban life. Or is it?
Across the globe, many countries use population as the scale to measure the level or urbanism, but that range varies from 2,000 in Argentina to 2,500 in the USA, to 20,000 in Nigeria. Urbanization is also defined on the basis of the population density, the administrative set ups, economic attributes, and urban characteristics in many countries. In India, cities are defined on the basis on population, but urban areas are determined on the basis of both population and occupation - 75% of adult working males have to be occupied in non-agricultural profession for the area to qualify as urban, even if it satisfies the population criteria.
Density can also be used a criteria - starting from the centre, till the point where density falls below the threshold limit can be considered a city. Density is crucial to the success of any city, because many urban services, be it BRTS bus systems or metro trains, require a certain density and footfall to be used efficiently. However, neither of these terms can fully define where a city truly ends - with the growth of many cities fueling exponential growth of suburbs and peri-urban areas that are dependent on the bigger city, where does a city truly end?
According to Sorokin and Zimmerman, there are 8 characters of what makes a city: occupation, environment, system of interaction, mobility, size of community, density and heterogeneity of population and social stratification. The academic definitions are many, but once again the question arises - what is urban, and what is a city? Is it just a checklist of multiple characteristics?
Of course, in the scale of urbanism, the city is neither the smallest nor the biggest, but its certainly the most important. It is the nucleus of this cell of urbanization, if you may. Sure, the Golgi bodies may be bigger, but this cell will fall apart without its nucleus.
The history of cities in ancient - it is the history of human settlements. Humans started settling in small peasant villages and practice agriculture and animal husbandry, and eventually, advances in animal breeding and cultivation allowed them to settle in larger populations. The land was able to sustain more people.
The invention of the wheel, and then of roads, and then of transporting water (at its finest under the Romans), truly made it possible to settle in a land for generations - giving rise to the Early Civilisations and some of the earliest cities. In the Greek city-state or Polis, the city was governed by prominent families, giving rise to the earliest stages of governments, administration and democracy.
Over time, cities gave opportunities for everyone to make wealth, as the country was ruled over by the lower orders of royalty. The country had landless labourers and serfs, but the city had guilds and merchants. From here on, the cities were seen as lands of opportunities.
Since then, the mercantile nature of cities has overthrown the monarchial, and brought in market friendly rule - now seen as democracy and a capitalist rule.
Why do cities form? Location. Always, location has been key. Whether to protect it from invaders, or locating it close to water, or locating it close to trade routes, people throng to a settlement when its location allows for benefits - for opportunities. Some of the key types of cities, to name a few, include:
Political cities - those that serve as administrative capitals and have been formed by political will. Brasilia, Chandigarh, and even New Delhi are examples of the same.
Economic cities - these are the economic bases for larger regions, sometimes even countries. They include trade, tertiary services and many more.
Trade cities - these cities sustain majorly on trade. Ports, transport interchanges and such are examples of this.
Tourism cities - these thrive on tourism. Spas, beaches, marvels of the world - these cities rely on tourists.
Religious cities - their main attraction is a place of religion. They also thrive on tourism, specific to religion.
These are not the only types of cities. These too, differ sharply as per the definition - and there are multiple classifications given by multiple researchers. In fact, cities have been the subject of multiple theories and rules over the years.
City life has always been more. It thrives on aspiration, on culture, on being the bright, shining light attracting everyone to it. Cities have jobs, cities have money, cities have opportunity - and that forms the crux of planning for it. To plan a city is to plan its opportunities, its attractions, its place in the economy of a country. The spatial aspect is but one layer of the city.
No matter if we talk of normative theories (such as the Central Place theory) or behavioral (like Zipf's Rank Size rule), or delve in particular studies of spatial growth - each city is unique in own way, with its concept, own drivers of growth, relationships to the hinterland and its location, purpose and stage of growth and migration.
Population, density and heterogeneity make the base of any city. Recently, there has been a push for many anti-urban concepts that are in direct contravention to what makes a city. "Smart cities", "tech cities" are buzz words nowadays, but they forget that the true essence of a city lies in its messiness and unpredictability. It is not an excuse to defend poorly planned cities, but the idea that everything in a city cannot and should not be dictated and monitored. A city thrives in its density, anonymity and opportunity, in its heterogeneity and diversity, and therein lies its charm - that a city is for everybody.