What is the X in X-mas?
mid-14c., "condition of being mortal," from Old French mortalite "massacre, slaughter; fatal illness; poverty; destruction" (12c.), from Latin mortalitem (nominative mortalitas) "state of being mortal; subjection to death," from mortalis (see mortal (adj.)). Meaning "widespread death" is from c.1400; meaning "number of deaths from some cause or in a given period" is from 1640s.
mortality mor·tal·i·ty (môr-tāl'ĭ-tē)
The quality or condition of being mortal.
in demographic usage, the frequency of death in a population