/ mɪˈleniəm; NAmE / noun ( pl. mil·len·nia / -niə; NAmE / or mil·len·niums )
1.
a period of 1 000 years, especially as calculated before or after the birth of Christ :
the second millennium AD
2.
the millennium the time when one period of 1 000 years ends and another begins :
How did you celebrate the millennium?
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WORD ORIGIN
mid 17th cent.: modern Latin , from Latin mille thousand, on the pattern of biennium .