(~s, or millennia)
1.
A ~ is a period of one thousand years, especially one which begins and ends with a year ending in ‘000’, for example the period from the year 1000 to the year 2000. (FORMAL)
N-COUNT
2.
Many people refer to the year 2000 as the Millennium.
...the eve of the Millennium.
N-SING: usu the N, N n