The Collaborative International Dictionary of English v.0.48:

Mortality \Mor*tal"i*ty\, n. [L. mortalitas: cf. F.
   mortalit['e].]
   1. The condition or quality of being mortal; subjection to
      death or to the necessity of dying.
      [1913 Webster]

            When I saw her die,
            I then did think on your mortality.   --Carew.
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   2. Human life; the life of a mortal being.
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            From this instant
            There 's nothing serious in mortality. --Shak.
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   3. Those who are, or that which is, mortal; the human race;
      humanity; human nature.
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            Take these tears, mortality's relief. --Pope.
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   4. Death; destruction. --Shak.
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   5. The whole sum or number of deaths in a given time or a
      given community; also, the proportion of deaths to
      population, or to a specific number of the population;
      death rate; as, a time of great, or low, mortality; the
      mortality among the settlers was alarming.
      [1913 Webster]

   Bill of mortality. See under Bill.

   Law of mortality, a mathematical relation between the
      numbers living at different ages, so that from a given
      large number of persons alive at one age, it can be
      computed what number are likely to survive a given number
      of years.

   Table of mortality, a table exhibiting the average relative
      number of persons who survive, or who have died, at the
      end of each year of life, out of a given number supposed
      to have been born at the same time.
      [1913 Webster]

WordNet (r) 3.0 (2006):

mortality
    n 1: the quality or state of being mortal [ant: immortality]
    2: the ratio of deaths in an area to the population of that
       area; expressed per 1000 per year [syn: deathrate, death
       rate, mortality, mortality rate, fatality rate]

Moby Thesaurus II by Grady Ward, 1.0:

61 Moby Thesaurus words for "mortality":
   Adam, Hominidae, Homo sapiens, banefulness, caducity,
   changeableness, clay, corruptibility, deadliness, death,
   death rate, death toll, ephemerality, ephemeralness, evanescence,
   fallen humanity, fatality, finitude, fleetingness, flesh, frailty,
   fugacity, generation of man, genus Homo, hominid, homo,
   human equation, human family, human frailty, human nature,
   human race, human species, human weakness, humanity, humankind,
   humanness, impermanence, impermanency, instability,
   le genre humain, lethality, malignance, malignancy, malignity, man,
   mankind, momentariness, mortal flesh, mortalness, mortals,
   mutability, perishability, perniciousness, race of man, transience,
   transiency, transientness, transitoriness, virulence, volatility,
   weakness