Source : Webster's Revised Unabridged Dictionary (1913)

Decimation \Dec`i*ma"tion\, n. [L. decimatio: cf. F.
   d['e]cimation.]
   1. A tithing. [Obs.] --State Trials (1630).

   2. A selection of every tenth person by lot, as for
      punishment. --Shak.

   3. The destruction of any large proportion, as of people by
      pestilence or war. --Milman.

Source : WordNet®

decimation
     n : destroying or killing a large part of the population
         (literally every tenth person as chosen by lot)