[dek-eyd; British also duh-keyd]
a period of ten years: the three decades from 1776 to 1806.
a period of ten years beginning with a year whose last digit is zero: the decade of the 1980s.
a group, set, or series of ten.

1425–75; late Middle English < Middle French < Late Latin decad- (stem of decas) < Greek dekad- (stem of dekás) group of ten, equivalent to dék(a) ten + -ad- -ade2

half-decade, noun Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2014.
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World English Dictionary
decade (ˈdɛkeɪd, dɪˈkeɪd)
1.  a period of ten consecutive years
2.  a group or series of ten
[C15: from Old French, from Late Latin decad-, decas, from Greek dekas, from deka ten]

Collins English Dictionary - Complete & Unabridged 10th Edition
2009 © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins
Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009
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Word Origin & History

mid-15c., "ten parts" (of anything; originally in ref. to the books of Livy), from M.Fr. decade, from L.L. decadem (nom. decas), from Gk. dekas (acc. dekada) "group of ten." Meaning "ten years" is 1590s in English.
Online Etymology Dictionary, © 2010 Douglas Harper
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Example sentences
But if it is to achieve the goal of development over the next decade, more
  reform will be needed.
We're now half a decade into mountain biking's dual-suspension dynasty.
Here's how much you'd have at the end of June if you had invested $100 a month
  into the five largest stock funds for a decade.
This publishing season kicks off a new decade—and things may never be the
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