The Collaborative International Dictionary of English v.0.48:

demography \de*mog"ra*phy\ (d[-e]*m[o^]g"r[.a]f[y^]), n. [Gr.
   dh^mos the people + -graphy.]
   The study of vital statistics, such as births, deaths,
   marriages, mortality, health, etc., in populations and
   subgroups of populations. -- Dem`o*graph"ic, a.
   [1913 Webster +PJC]

WordNet (r) 3.0 (2006):

demography
    n 1: the branch of sociology that studies the characteristics of
         human populations [syn: demography, human ecology]