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]

Moby Thesaurus II by Grady Ward, 1.0:

65 Moby Thesaurus words for "decade":
   Big Dick, Decalogue, Ten Commandments, X, abundant year,
   academic year, annum, bissextile year, calendar month,
   calendar year, century, common year, day, decagon, decagram,
   decahedron, decaliter, decameter, decare, decastere, decastyle,
   decasyllable, decemvir, decemvirate, decennary, decennium,
   decigram, deciliter, decimeter, decurion, defective year,
   fiscal year, fortnight, hour, leap year, lunar month, lunar year,
   lunation, luster, lustrum, man-hour, microsecond, millennium,
   millisecond, minute, moment, month, moon, quarter, quinquennium,
   regular year, second, semester, session, sidereal year, solar year,
   sun, ten, tenner, term, trimester, twelvemonth, week, weekday,
   year