The density

Istanbul (Turkish: İstanbul) is the largest city in Turkey, constituting the country's economic, cultural, and historical heart. With a population of 13.9 million, the city forms one of the largest urban agglomerations in Europe and is among the largest cities in the world by population within city limits. Istanbul's vast area of 5,343 square kilometers (2,063 sq mi) is coterminous with Istanbul Province, of which the city is the administrative capital. Istanbul is a transcontinental city, straddling the Bosphorus—one of the world's busiest waterways—in northwestern Turkey, between the Sea of Marmara and the Black Sea. Its commercial and historical center lies in Europe, while a third of its population lives in Asia.

Founded on the Sarayburnu promontory around 660 BC as Byzantium, the city now known as Istanbul developed to become one of the most significant cities in history. For nearly sixteen centuries following its reestablishment as Constantinople in 330 AD, it served as the capital of four empires: the Roman Empire (330–395), the Byzantine Empire (395–1204 and 1261–1453), the Latin Empire (1204–1261), and the Ottoman Empire (1453–1922). It was instrumental in the advancement of Christianity during Roman and Byzantine times, before the Ottomans conquered the city in 1453 and transformed it into an Islamic stronghold and the seat of the last caliphate. Although the Republic of Turkey established its capital in Ankara, palaces and imperial mosques still line Istanbul's hills as visible reminders of the city's previous central role.

A city is a relatively large and permanent settlement. Although there is no agreement on how a city is distinguished from a town within general English language meanings, many cities have a particular administrative, legal, or historical status based on local law.

For example, in the American state of Massachusetts an article of incorporation approved by the local state legislature distinguishes a city government from a town. In the United Kingdom and parts of the Commonwealth of Nations, a city is usually a settlement with a royal charter. Historically, in Europe, a city was understood^{[citation needed]} by some to mean an urban settlement with a cathedral. The belief in this distinction is also common in England, where the presence of a cathedral is thought by many to distinguish a 'city' (sometimes called a 'cathedral city') from a 'town' (which has a parish church^{[citation needed]}); the belief is incorrect (Chelmsford, for example, became a city only in 2012, but had a cathedral for most of the 20^th century).

An urban area is characterized by higher population density and vast human features in comparison to areas surrounding it. Urban areas may be cities, towns or conurbations, but the term is not commonly extended to rural settlements such as villages and hamlets.

Urban areas are created and further developed by the process of urbanization. Measuring the extent of an urban area helps in analyzing population density and urban sprawl, and in determining urban and rural populations.^{[citation needed]}

Unlike an urban area, a metropolitan area includes not only the urban area, but also satellite cities plus intervening rural land that is socio-economically connected to the urban core city, typically by employment ties through commuting, with the urban core city being the primary labor market.

In the US, Metropolitan areas tend to be defined using counties or county sized political units as building blocks of much larger, albeit more condensed population units. Counties tend to be stable political boundaries; economists prefer to work with economic and social statistics based on metropolitan areas. Urbanized areas are a more relevant statistic for determining per capita land usage and densities.^{[citation needed]}

Population density (in agriculture standing stock and standing crop) is a measurement of population per unit area or unit volume. It is frequently applied to living organisms, and particularly to humans. It is a key geographic term.

Urbanization is the physical growth of urban areas as a result of rural migration and even suburban concentration into cities, particularly the very large ones. The United Nations projected that half of the world's population would live in urban areas at the end of 2008. By 2050 it is predicted that 64.1% and 85.9% of the developing and developed world respectively will be urbanised.

Urbanization is closely linked to modernisation, industrialisation, and the sociological process of rationalisation. Urbanisation can describe a specific condition at a set time, i.e. the proportion of total population or area in cities or towns, or the term can describe the increase of this proportion over time. So the term urbanisation can represent the level of urban relative to overall population, or it can represent the rate at which the urban proportion is increasing.

Urbanization is not merely a modern phenomenon, but a rapid and historic transformation of human social roots on a global scale, whereby predominantly village culture is being rapidly replaced by predominantly urban culture. The last major change in settlement patterns was the accumulation of hunter-gatherers into villages many thousand years ago. Village culture is characterised by common bloodlines, intimate relationships, and communal behavior whereas urban culture is characterised by distant bloodlines, unfamiliar relations, and competitive behavior. This unprecedented movement of people is forecast to continue and intensify in the next few decades, mushrooming cities to sizes incomprehensible only a century ago. Indeed, today, in Asia the urban agglomerations of Dhaka, Karachi, Mumbai, Delhi, Manila, Seoul and Beijing are each already home to over 20 million people, while the Pearl River Delta, Shanghai-Suzhou and Tokyo are forecast to approach or exceed 40 million people each within the coming decade. Outside Asia, Mexico City, Sao Paulo, New York City, Lagos and Cairo are fast approaching or home to over 20 million people already.

Overpopulation is a generally undesirable condition where an organism's numbers exceed the current carrying capacity of its habitat. The term often refers to the relationship between the human population and its environment, the Earth, or smaller geographical areas such as countries. Overpopulation can result from an increase in births, a decline in mortality rates, an increase in immigration, or an unsustainable biome and depletion of resources. It is possible for very sparsely populated areas to be overpopulated if the area has a meager or non-existent capability to sustain life (e.g. a desert).

The population has been growing continuously since the end of the Black Death, around the year 1400,^{[citation needed]} although the most significant increase has been in the last 50 years, mainly due to medical advancements and increases in agricultural productivity. Although the rate of population growth has been declining since the 1980s, the United Nations has expressed concern on continued excessive population growth in sub-Saharan Africa. As of April 25, 2013 the world's human population is estimated to be 7.081 billion by the United States Census Bureau, and over 7 billion by the United Nations. Most contemporary estimates for the carrying capacity of the Earth under existing conditions are between 4 billion and 16 billion. Depending on which estimate is used, human overpopulation may or may not have already occurred. Nevertheless, the rapid recent increase in human population is causing some concern. The population is expected to reach between 8 and 10.5 billion between the year 2040 and 2050. In May 2011, the United Nations increased the medium variant projections to 9.3 billion for 2050 and 10.1 billion for 2100.

The Golden Horn (Turkish: Haliç (which is derived from the Arabic word Khaleej, meaning Gulf) or Altın Boynuz (literally "Golden Horn" in Turkish); Greek: Κεράτιος Κόλπος, Keratios Kolpos: Horn-shape gulf) is an inlet of the Bosphorus dividing the city of Istanbul and forming the natural harbor that has sheltered Greek, Roman, Byzantine, Ottoman and other ships for thousands of years. It is a scimitar-shaped estuary that joins the Bosphorus just at the point where that strait enters the Sea of Marmara, thus forming a peninsula the tip of which is "Old Istanbul" (ancient Byzantion and Constantinople). Its Greek and English names mean the same, but the significance of the designation "golden" is obscure, while its Turkish name Haliç simply means "estuary". It has witnessed many tumultuous historical incidents, and its dramatic vistas have been the subject of countless works of art.