Mortality is the susceptibility to death (‘mortal’). When actuaries refer to mortality they generally are concerned with the likelihood of death. The mortality rate, or ‘death rate’, is a measure of the number of deaths in a given population. The rate can be defined in a number of different ways. The most commonly used definition by actuaries is the probability of dying within the following year. For example, the mortality rate for someone aged 70 (exactly) is the probability that he or she will die before they reach their 71st birthday.