1.NounA period of time consisting of one thousand years.
2.NounThe year in which one period of one thousand years ends and another begins, especially the year 2000.
"A huge fireworks display was put on in Sydney to celebrate the millennium."
3.NounAny long (and possibly endless) era of universal happiness, peace, and prosperity, predicted by various people to come about someday.
"expecting a millennium"
4.Proper nounThe period of one thousand years during which Christ will reign on earth (according to Millenarianist interpretations).
"usher in the millennium; The Nazis' concept for the so-called Thousand-Year Reich of their infallible Leader has been described as but a pale pseudoreligious false-prophet imitation of the Christian concept of the millennium of Christ regnant."
5.Proper nounA particular long (and possibly endless) era of universal happiness, peace, and prosperity, predicted by various people to come about someday.