Fertility is the property of a population to increase its number by reproduction. A distinction is made between maximum (potential) and actual (ecological) fertility.

Fertility is usually estimated by the number of individuals born in a population per unit of time relative to a conventional number of individuals (100 or 1000). Sometimes a specific estimate of fertility is used, i.e. per individual or per female in the population. Fertility is higher the greater the proportion of individuals taking part in reproduction, the higher the fecundity, the more frequent the successive reproductive cycles. Usually, fertility in each population is balanced by its characteristic mortality.