The difference between an individual's life expectancy and his or her age at death.

Life expectancy is a generalization for a specific generation or peoples in a given era. People born between the late 90s to early 2000s have a greater live expectancy than say peoples 300 years ago. We are “expected” to live until about our late 70s, early 80s because we as a society have the means to keep us living longer. Someone who dies before that window, may have had some complications with their health that prevented them the estimated age.