noun pl. millenniums or millennia
- any period of 1,000 years: 2000 through 1001 is the 2d millennium
- 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
- a 1,000th anniversary or its commemoration
- any period of great happiness, peace, prosperity, etc.; imagined golden age
Origin:
ModL < L mille, thousand + annus, year (after L biennium): see annual