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)]

millennia
     See {millennium}