noun pl. millenniums -·ni·ums or millennia -·nia (-ə)

1. any period of 1,000 years 2000 through 1001 is the 2d millennium
2. a period of 1,000 years reckoned from a certain time, esp. from the beginning of the Christian Era (1) the year A.D. 2001 marks the beginning of the third millennium
3. a 1,000th anniversary or its commemoration
4. any period of great happiness, peace, prosperity, etc.; imagined golden age

Etymology: ModL < L mille, thousand + annus, year (after L biennium): see annual