Population density is a measurement of population

Population

A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

 per unit area or unit volume. It is frequently applied to living organisms, and particularly to human

Human

Humans are the only living species in the Homo genus...

s. It is a key geographic term.
Population density is population divided by total land area.

Low densities may cause an extinction vortex

Extinction Vortex

Extinction vortices are a class of models through which conservation biologists, geneticists and ecologists can understand the dynamics of and categorize extinctions in the context of their causes. Developed by M. E. Gilpin and M. E...

 and lead to further reduced fertility. This is called the Allee effect

Allee effect

The Allee effect is a phenomenon in biology characterized by a positive correlation between population density and the per capita population growth rate in very small populations.-Description:...

 after the scientist who identified it.