A primate city is the leading city in its country or region, disproportionately larger than any others in the urban hierarchy

Urban hierarchy

Urban hierarchy a term that relates the structure of towns within an area. It can typically be illustrated by dividing towns into four categories:* 1st-order towns* 2nd-order towns* 3rd-order towns* 4th-order towns...

. A 'primate city distribution' has one very large city with many much smaller cities and towns, and no intermediate-sized urban centres, in contrast to the linear 'rank-size distribution

Rank-size distribution

Rank-size distribution or the rank-size rule describes the remarkable regularity in many phenomena including the distribution of city sizes around the world, sizes of businesses, particle sizes , lengths of rivers, frequencies of word usage, wealth among individuals, etc...

'. The 'law of the primate city' was first proposed by the geographer

Geographer

A geographer is a scholar whose area of study is geography, the study of Earth's natural environment and human society.Although geographers are historically known as people who make maps, map making is actually the field of study of cartography, a subset of geography...

 Mark Jefferson

Mark Jefferson (geographer)

Mark Jefferson was the chief cartographer of the American Delegation to the Paris Peace Conference in 1919. He was also the head of the geography department at Michigan State Normal College , now Eastern Michigan University , from 1901-1939.Jefferson received his bachelors degree from Boston...

 in 1939. He defines a primate city as being "at least twice as large as the next largest city and more than twice as significant." A primate city is number one in its country in most aspects, like politics, economy, media, culture and universities.
Not all countries have primate cities, but in those that do, the rest of the country depends on it for cultural, economic, political, and major transportation needs.