Definition from Wiktionary, the free dictionary
From Latin decimātiō, a punishment where every 10th man in a unit would be stoned to death by the men who were spared. Used by the Romans to keep order in their military. Compare septimation and vicesimation.
decimation (plural decimations)
- The killing or destruction of a large portion of a population.
- 1702: Cotton Mather, Magnalia Christi Americana - And the whole army had cause to enquire into their own rebellions, when they saw the Lord of Hosts, with a dreadful decimation, taking off so many of our brethren by the worst of executioners.
- A tithing.
- A selection of every tenth person by lot, as for punishment.
- (mathematics) The creation of a new sequence comprising only every nth element of the original sequence.
- (telecommunications) A digital signal processing technique for reducing the number of samples in a discrete-time signal.
Coordinate terms 
- (proportionate reduction, by single aliquot part): quintation (⅕), septimation (⅐), vicesimation (¹⁄₂₀), tricesimation (¹⁄₃₀), centesimation (¹⁄₁₀₀)
tithing — see tithing
selection of every tenth person for punishment
math: creation of new sequence by taking every n'th element of original
digital signal processing technique
- The Century Dictionary, The Century Co., New York, 1914