The Collaborative International Dictionary of English v.0.48:

Decade \Dec"ade\, n. [F. d['e]cade, L. decas, -adis, fr. Gr. ?,
   fr. de`ka ten. See Ten.]
   A group or division of ten; esp., a period of ten years; a
   decennium; as, a decade of years or days; a decade of
   soldiers; the second decade of Livy. [Written also decad.]
   [1913 Webster]

         During this notable decade of years.     --Gladstone.
   Decadence

WordNet (r) 3.0 (2006):

decade
    n 1: a period of 10 years [syn: decade, decennary,
         decennium]
    2: the cardinal number that is the sum of nine and one; the base
       of the decimal system [syn: ten, 10, X, tenner,
       decade]