Historically, the term comes from the presence of a defensive fence around the settlement - a rampart or wall. In Ancient Russia, any large residential place surrounded by such a fence was called a city. Cities served as a center for the development of arts and crafts, technical achievements.
Growing, cities form urban agglomerations. Especially important for countries and continents are capitals, as well as million-plus cities or million-plus agglomerations (having a population of more than 1 million people), including megalopolises and global cities.
The study of the correct organization of the city is engaged in a special discipline - urban planning, which studies the laws and rules for the organization of cities. Urban planning was born in the bowels of architecture, but over time, cities became large, and their planning required additional knowledge - about hygiene, economics, ecology, transport, and much more.