Life expectancy
This is used above all as a measure of mortality. It takes into account the age distribution and size of the population, which enables comparisons between different populations and makes it suitable as a general measure of the health status of a population.
Life expectancy period calculations are based on the mortality rates for all ages in a given year. This means it cannot be seen as equal to longevity, since mortality rates change throughout a lifetime. A decrease in death rates leads to a higher cohort life expectancy - which can be seen as longevity - as opposed to the life expectancy period.