The Collaborative International Dictionary of English v.0.48:

Decimate \Dec"i*mate\, v. t. [imp. & p. p. Decimated; p. pr. &
   vb. n. Decimating.] [L. decimatus, p. p. of decimare to
   decimate (in senses 1 & 2), fr. decimus tenth. See
   Decimal.]
   1. To take the tenth part of; to tithe. --Johnson.
      [1913 Webster]

   2. To select by lot and punish with death every tenth man of;
      as, to decimate a regiment as a punishment for mutiny.
      --Macaulay.
      [1913 Webster]

   3. To destroy a considerable part of; as, to decimate an army
      in battle; to decimate a people by disease.
      [1913 Webster]

WordNet (r) 3.0 (2006):

decimate
    v 1: kill one in every ten, as of mutineers in Roman armies
    2: kill in large numbers; "the plague wiped out an entire
       population" [syn: eliminate, annihilate, extinguish,
       eradicate, wipe out, decimate, carry off]