Definition of "Millennium" :

noun: a span of 1000 years

noun: the 1000th anniversary (or the celebration of it)

noun: (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