birth rate See crude birth rate.

crude birth rate Annual number of live births per 1,000 people in the population of a geographic area at the midpoint of a given year. Compare crude death rate. 

crude death rate Annual number of deaths per 1,000 people in the population of a geographic area at the midpoint of a given year. Compare crude birth rate. 

death rate See crude death rate. 

Demographic transition Hypothesis that countries, as they become industrialized, have declines in death rates followed by declines in birth rates. 

family planning Providing information, clinical services, and contraceptives to help people choose the number and spacing of children they want to have.

infant mortality rate Number of babies out of every 1,000 born each year that die before their first birthday.

life expectancy Average number of years a newborn infant can be expected to live. 

linear growth Growth in which a quantity increases by some fixed amount during each unit of time. Compare exponential growth.

Population change Increase or decrease in the size of a population. It is equal to (Births + Immigration) - (Deaths + Emigration). 

Population density Number of organisms in a particular population found in a specified area or volume. 

Population distribution Variation of population density over a particular geographic area. For example, a country has a high population density in its urban areas and a much lower population density in rural areas.

Replacement level fertility Number of children a couple must have to replace them. The average for a country or the world usually is slightly higher than 2 children per couple (2.1 in the United States and 2.5 in some developing countries) because some children die before reaching their reproductive years. See also total fertility rate. 

total fertility rate (TFR) Estimate of the average number of children who will be born alive to a woman during her lifetime if she passes through all her childbearing years (ages 15-44) conforming to age-specific fertility rates of a given year. In simpler terms, it is an estimate of the average number of children a woman will have during her childbearing years.

zero population growth (ZPG) State in which the birth rate (plus immigration) equals the death rate (plus emigration) so the population of a geographic area is no longer increasing.