One of the most difficult things for a child is experiencing the death of a beloved pet; it's a hard lesson in mortality. Mortality is the quality of being mortal, or the reality of death.
It's not easy for most people to come to terms with their own mortality, or the fact that they really will die someday. Mortality can refer to an individual or to a larger group of people, particularly when you're talking about the total number of deaths within a population, using the phrase mortality rate. The words mortality and mortal come from the Latin root mortis, or "death."