Mortality is best defined as:

Mortality is defined as the frequency or rate of death in a specific population, typically over a designated period of time. This measurement is crucial in epidemiology and public health as it provides insight into the impact of diseases and health interventions on a community or population.

When analyzing public health data, understanding mortality helps in identifying the deadliness of certain conditions and how they affect different populations. It serves as an essential indicator of the overall health of a community, reflecting how many individuals are succumbing to diseases compared to how many are living with them.

In contrast, severity of disease pertains to how serious or intense a disease can be, but does not directly measure deaths. The measure of infection relates more to how many individuals are infected with a disease rather than the death outcome. Finally, the rate of new cases refers to incidence, which tracks how many new instances of a disease occur, but again does not specifically address deaths that result from those diseases. Thus, mortality stands out as the correct definition focusing specifically on death rates.