Source : Webster's Revised Unabridged Dictionary (1913)

Decennium \De*cen"ni*um\, n.; pl. {Decenniums}, L. {Decennia}.
   [L.]
   A period of ten years. ``The present decennium.'' --Hallam.
   ``The last decennium of Chaucer's life.'' --A. W. Ward.

Source : WordNet®

decennium
     n : a period of 10 years [syn: {decade}, {decennary}]
     [also: {decennia} (pl)]