The concept of a stable population in demography was first defined by A.J. Lotka. We say that a population is stable if the following three conditions are satisfied:

- The age and sex distribution of the population remains constant.
- The birth rate and death rate for each age remains constant.
- There is no immigration into or emigration out of the population.

Since the birth and death rates of the population are constant, therefore the population grows at a fixed rate. This is known as Malthus Law for population growth.

The concept of stable population must not be confused with the concept of stationary population. In a stationary population, the size of the population remains the same whereas, in a stable population the population size increases at a constant rate.