mil ‧ len ‧ ni ‧ um /mɪˈleniəm/ BrE AmE noun ( plural millennia /-niə/) [countable]
[ Date: 1600-1700 ; Language: Modern Latin ; Origin: Latin mille 'thousand' + annus 'year' ]
1 . a period of 1,000 years:
people who have inhabited this land for millennia
2 . [usually singular] the time when a new 1,000-year period begins:
the beginning of a new millennium
events which took place at the turn of the last millennium
—millennial adjective