(If you were linked from a country article: the density there is based on land area, see below)

Population density can be used as a measurement of any tangible item. However it is most frequently applied to living organisms. Population density is usually expressed in terms of items or organisms per unit area.

Table of contents
1 How to define population density?
2 Biological population densities
3 Human population density

How to define population density?

Note that population density, according to this definition, depends on the scale of the sampling area used, and is hard to define as a real-valued continuous function over the area in question. If the items concerned are modelled as discrete points, the population density will jump up and down as the edge of the sampling area passes over individuals. Modelling the individuals as spatially extended objects has other problems, as the scale of the sampling area approaches the scale of an object (for example, a person's scale may be regarded as the size of the grounds of their dwelling place: for some people, this will be a large area).

To resolve some of these problems, population density may be regarded, like coastline distance, as a scale-dependent fractal quantity.

Biological population densities

Population density is a common biological measurement and is often used by conservationists as a more appropriate measure than absolute numbers. Low population densities may cause an extinction vortex, where low densities lead to further reduced fertility. This is referred to as the Allee effect, named after W. C. Allee, who first identified it. Examples of this may include;

1. Increased problems with locating mates in areas of low density.
2. Increased inbreeding in areas of low population density.
3. Increased susceptibility to catastrophic events in low population densities.

However it should be noted that different species will have different expected densities. For example r selected species commonly have high population densities, while k selected species may have lower population densities. Low population densities may be associated with specialised mate location adaptations such as specialised pollinators, as is found in the orchid family, (the Orchidaceae.)

Human population density

For human beings, population density is the number of persons per unit of area (which may include or exclude inland water), though it may also be expressed in relation to habitable, inhabited, productive (or potentially productive) or cultivated area.

It is frequently measured in persons per square mile or persons per square kilometre or hectare, which can be obtained simply by dividing the number of persons by the land area measured in square miles or in square kilometers or hectares.

Commonly this may be calculated for a county, city, country or the entire world. In the country articles the density is based on land area. However, the list of countries by population density is based on total area, including inland water.

Countries or territories with the highest population densities are:

These territories share a relatively small area and an exceptionally high urbanization level, with an economically specialized city population drawing also on rural resources outside the area, illustrating the difference between high population density and overpopulation.

The most densely populated large state is Bangladesh, where 134 million people live in a highly agricultural area around the lower Ganges river, with a national population density in excess of 900 persons per kmē. World overall population density presently averages 42 persons per kmē.

Cities with exceptionally high population densities are often considered to be overpopulated, though the extent to which this is the case depends on factors like quality of housing and infrastructure or access to resources. Most of the largest densely-populated cities are in southern and eastern Asia, though Cairo and Lagos in Africa also fall into the category.

City population is, however, heavily dependent on the definition used for the urban area: densities will be far higher for the central municipality than when more recently-developed and as yet administratively unincorporated suburbs are included, as in the concepts of agglomeration or metropolitan area, the latter including sometimes neighbouring cities.

[http://www.demographia.com/db-citydenshist.htm Selected Current and Historic City, Ward & Neighborhood Densities]