The Collaborative International Dictionary of English v.0.48:

Decimation \Dec`i*ma"tion\, n. [L. decimatio: cf. F.
   d['e]cimation.]
   1. A tithing. [Obs.] --State Trials (1630).
      [1913 Webster]

   2. A selection of every tenth person by lot, as for
      punishment. --Shak.
      [1913 Webster]

   3. The destruction of any large proportion, as of people by
      pestilence or war. --Milman.
      [1913 Webster]

WordNet (r) 3.0 (2006):

decimation
    n 1: destroying or killing a large part of the population
         (literally every tenth person as chosen by lot)

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

DECIMATION. The punishment of every tenth soldier by lot, was, among the
Romans, called decimation.