Number of of a given species found per unit area or instead per unit volume.

It is the ratio of the density of organisms to the density of that determines a population's potential to or even persist at a given density. It also determines the level of impact that a population can have on its environment, particularly negative impact (see ).

The population density of prey organisms is a key component of the fitness of predator organisms (see also exploiter generally).

Similarly, host population density is a crucial determinant of the epidemiology of infectious diseases. Indeed, epidemics associated with communicable pathogens are possible only given sufficient densities of susceptible .

Population density is a key component of logistic growth and the associated concept of environmental carrying capacity. See also density dependent, density independent, and concentration.