Population density is a term that is used to refer to the number of individuals living in an area, in relation to the size of that area. It is simply the measurement of population per unit area. It is calculated by dividing the number of people to the area they occupy.
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Population density is the measure of how many people, i.e. the size of the population, that live within a square mile, or kilometer, of land. You can find more information here: en.wikipedia.org/wiki/Population_density