means The condition of being subject to death; also the rate or number of deaths in a given group or place.

from From Latin mortalitas, built on mortalis 'subject to death,' from mors, mortis 'death' — the same dark root that seeds mortal, mortgage (literally a 'death-pledge'), mortify, and the embalmer's mortuary. It reached English through Old French mortalite in the medieval period, and gained grim everyday use in the centuries of plague, when 'the mortality' was simply what people called the dying. The deeper ancestor is the Proto-Indo-European root *mer- 'to die,' a thread running through many languages.