Yes, MILLENNIUM is a Scrabble word!

Letter values in Scrabble US: M 3, I 1, L 1, L 1, E 1, N 1, N 1, I 1, U 1, M 3.

Is it a valid word?

MILLENNIUM is a valid word in the official Scrabble dictionary (US), the UK Scrabble Dictionary and Words with Friends (ENABLE).

a span of 1000 years
(New Testament) in Revelations it is foretold that those faithful to Jesus will reign with Jesus over the earth for a thousand years; the meaning of these words have been much debated; some denominations (e.g. Jehovah's Witnesses) expect it to be a thousand years of justice and peace and happiness
the 1000th anniversary (or the celebration of it)