mor·tal·i·ty
   /mɔrˈtælɪti/ [mawr-tal-i-tee]
–noun, plural -ties.
1. the state or condition of being subject to death; mortal character, nature, or existence.
2. the relative frequency of deaths in a specific population; death rate.
3. mortal beings collectively; humanity.
4. death or destruction on a large scale, as from war, plague, or famine.
5. Obsolete . death.

Origin:
1300–50;  ME mortalite  < MF < L mortālitās. See mortal, -ity
Dictionary.com Unabridged
 
— n  , pl -ties
1.     the condition of being mortal
2.     great loss of life, as in war or disaster
3.     the number of deaths in a given period
4.     mankind; humanity
5.     an obsolete word for death

Collins English Dictionary - Complete & Unabridged 10th Edition

mortality
mid-14c., "condition of being mortal," from Fr. mortalité  (12c.), from L. mortalitem , from mortalis  (see mortal). Meaning "widespread death" is from c.1400; meaning "number of deaths from some cause or in a given period" is from 1640s.
Online Etymology Dictionary, © 2010 Douglas Harper
 
Medical Dictionary

mor·tal·i·ty
Pronunciation:  /mȯr-ˈtal-ət-ē/
Function: n
pl -ties ; 1 :  the quality or state of being mortal
2 a :  the number of deaths in a given time or place
b :  the proportion of deaths to population : DEATH RATE called also mortality rate compare FERTILITY 2 , MORBIDITY 2 ,

Merriam-Webster's Medical Dictionary, © 2007 Merriam-Webster, Inc.

mortality  mor·tal·i·ty (môr-tāl'ĭ-tē)
n.

   1. The quality or condition of being mortal.
   2. Death rate.

The American Heritage® Stedman's Medical Dictionary

mortality

in demographic usage, the frequency of death in a population

Encyclopedia Britannica, 2008. Encyclopedia Britannica Online.