Life expectancy is a statistical measure of how long a person may live, based on the year of their birth, their current age and other demographic factors including gender. At a given age (age x) is the average number of years that would be lived by a group of individuals (of age x) exposed to the same mortality conditions until they die. The most commonly used measure of life expectancy is life expectancy at age zero, that is, at birth (LEB), which can be defined in two ways: while cohort LEB is the mean length of life of an actual birth cohort (all individuals born a given year) and can be computed only for cohorts that were born many decades ago, so that all their members died, period LEB is the mean length of life of a hypothetical cohort assumed to be exposed since birth until death of all their members to the mortality rates observed at a given year.
National LEB figures reported by statistical national agencies and international organizations are indeed estimates ofperiod LEB. In the Bronze and Iron Age LEB was 26 years; the 2010 world LEB was 67.2. For recent years in Swaziland LEB is about 49 years while in Japan is about 83 years. The combination of high infant mortality and deaths in young adulthood from accidents, epidemics, plagues, wars, and childbirth, particularly before modern medicine was widely available, significantly lowers LEB. But for those who survive early hazards, a life expectancy of sixty or seventy would not be uncommon. For example, a society with a LEB of 40 may have few people dying at age 40: most will die before 30 years of age or very few after 55. In countries with high infant mortality rates, LEB is highly sensitive to the rate of death in the first few years of life. Because of this sensitivity to infant mortality, LEB can be subjected to gross misinterpretation, leading one to believe that a population with a low LEB will necessarily have a small proportion of older people. For example, in a hypotheticalstationary population in which half the population dies before the age of five, but everybody else dies at exactly 70 years old, LEB will be about 36 years, while about 25% of the population will be between the ages of 50 and 70. Another measure, such as life expectancy at age 5 (e5), can be used to exclude the effect of infant mortality to provide a simple measure of overall mortality rates other than in early childhood—in the hypothetical population above, life expectancy at age 5 would be another 65 years. Aggregate population measures, such as the proportion of the population in various age groups, should also be used alongside individual-based measures like formal life expectancy when analyzing population structure and dynamics.
Mathematically, life expectancy is the expected (in the statistical sense) number of years of life remaining at a given age. It is denoted by 
, which means the averagenumber of subsequent years of life for someone now aged 
, according to a particular mortality experience. Because life expectancy is an average, a particular person may well die many years before or many years after their "expected" survival. The term "maximum life span" has a quite different meaning.
, which means the averagenumber of subsequent years of life for someone now aged , according to a particular mortality experience. Because life expectancy is an average, a particular person may well die many years before or many years after their "expected" survival. The term "maximum life span" has a quite different meaning.
Life expectancy is also used in plant or animal ecology; life tables (also known asactuarial tables). The term life expectancy may also be used in the context of manufactured objects, although the related term shelf life is used for consumer products and the terms "mean time to breakdown" (MTTB) and "mean time between failures" (MTBF) are used in engineering.
