Definition of millennium
1a : the thousand years mentioned in Revelation (see revelation sense 3) 20 during which holiness is to prevail and Christ is to reign on earth
b : a period of great happiness or human perfection
2a : a period of 1000 years especially : one reckoned from the beginning of the Christian era at the start of the third millennium
b : a 1000th anniversary or its celebration In 2015 the city of Leipzig, Germany, celebrated its millennium.
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