Hi Abbie,
The ratio of the total number of deaths to the total population. Can also refer to the rate of death from a particular disease. e.g. SARS has almost 100% mortality. ”
If I'm not mistaken, mortality rate (as in infant mortality rate, maternal mortality rate) is calculated as the ratio of deaths to births (or births + deaths) in a given period (not always, but very often)
Mortality itself isn't a rate.
From Oxford Phrasal Verbs Dictionary for learners of English:
1 [U ] the state of being human and not living for ever: After her mother's death, she became acutely aware of her own mortality.
2 [U ] the number of deaths in a particular situation or period of time: the infant mortality rate (= the number of babies that die at or just after birth) Mortality from lung cancer is still increasing.
3 [C ] (technical) a death: hospital mortalities (= deaths in hospital)
I could very well be wrong though.
Thanks!