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For the use of the word population in statistics, see statistical population.

In the most common sense of the word, a population is the collection of people, or organisms of a particular species, living in a geographic area.

Demography studies the size, age and sex structure, mortality, reproductive behaviour and growth of human populations. Other aspects are studied in sociology, economics and geography. Plant or animal populations are studied in biology and in particular, population biology, a branch of ecology, and population genetics.

In biology, population denotes a breeding group, whose members breed solely among themselves, for instance through physical isolation, though biologically they could breed with all members of the species or subspecies.

Population density measures the number of people or organisms per unit of area. Variants may express the population per unit of habitable, inhabited, productive (or potentially productive) or cultivated area.

A particular geographic area of land is said to have a carrying capacity, representing the maximum population which it can support. Some observers of human societies believe that the concept of carrying capacity also applies to human population, and that unchecked population growth can result in a Malthusian catastrophe. Others oppose this view.

Population may also mean the process of populating a geographic area, as by procreation or immigration.