The Collaborative International Dictionary of English v.0.48:

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.
   Decennoval

WordNet (r) 3.0 (2006):

decennium
    n 1: a period of 10 years [syn: decade, decennary,
         decennium]