We all talk about cities and many of us live in one, but what exactly is a city? We often think of the city as a "modern" or recent development, but cities have existed for thousands of years and have their roots in the great river valley civilizations of Mesopotamia, Egypt, India and China. The English word comes from the Latin "civitas", which describes a highly organized community like the city-states of Ancient Greece.
Although we might know or sense what a city is, there is no exact definition of its boundaries, of where it starts and where it ends. In the past, walls may have defined a city. In many ancient cities you can still see the ruins of those walls, but the walls no longer mean anything. How do you define a city today? Do you include all the outlying areas and suburbs (called the "metropolitan region") or do you only include the city centre? Questions like this can cause inaccuracies and disagreements. For example, depending on the boundaries used, Tokyo, Japan, can have a population of anywhere between 13 and 36 million.
No matter how you define a city, however, there is agreement that cities play an important role in all our lives today and in the years ahead. After the industrial revolution, urban centers grew rapidly and over the past 50 years there has been an "explosion" in the growth of cities, both in their numbers and in their size -- this is called "urbanization". Today, the most rapid urbanization is taking place in countries in Asia, Latin America and Africa.
Cities have always been at the center of economic growth, technological advances and cultural production. But their rapid growth has also brought negative things: urban violence and poverty, homelessness, overcrowding and health problems, pollution and waste.
Cities have had a great impact on our lives and on world civilization in general. They are becoming more and more important as their sizes and numbers grow. As of April 2010, more than half of the world's population is now living in cities.