Source : Webster's Revised Unabridged Dictionary (1913)

Millennium \Mil*len"ni*um\ (m[i^]l*l[e^]n"n[i^]*[u^]m), n. [LL.,
   fr. L. mille a thousand + annus a year. See {Mile}, and
   {Annual}.]
   A thousand years; especially, the thousand years mentioned in
   the twentieth chapter in the twentieth chapter of Revelation,
   during which holiness is to be triumphant throughout the
   world. Some believe that, during this period, Christ will
   reign on earth in person with his saints.

Source : WordNet®

millennium
     n 1: a span of 1000 years
     2: (New Testament) in Revelations it is foretold that those
        faithful to Jesus will reign with Jesus over the earth for
        a thousand years; the meaning of these words have been
        much debated; some denominations (e.g. Jehovah's
        Witnesses) expect it to be a thousand years of justice and
        peace and happiness
     3: the 1000th anniversary (or the celebration of it)
     [also: {millennia} (pl)]