The Collaborative International Dictionary of English v.0.48:

Depopulation \De*pop`u*la"tion\, n. [L. depopulatio pillaging:
   cf. F. d['e]population depopulation.]
   The act of depopulating, or condition of being depopulated;
   destruction or explusion of inhabitants.
   [1913 Webster]

         The desolation and depopulation [of St.Quentin] were
         now complete.                            --Motley.
   [1913 Webster]

WordNet (r) 3.0 (2006):

depopulation
    n 1: the condition of having reduced numbers of inhabitants (or
         no inhabitants at all)

Bouvier's Law Dictionary, Revised 6th Ed (1856):

DEPOPULATION. In its most proper signification, is the destruction of the
people of a country or place. This word is, however, taken rather in a
passive than an active one; we say depopulation, to designate a diminution
of inhabitants, arising either from violent causes, or the want of
multiplication. Vide 12 Co. 30.