a span of 1000 years
"The ancient civilization flourished during that millennium, spanning ten centuries from its founding to its fall."
(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
"According to Revelation, the faithful are foretold to reign with Jesus over the earth for this millennium, a period many interpret as an era of perfect justice and peace."
the 1000th anniversary (or the celebration of it)
"The city held a grand parade to celebrate the millennium."
A period of time consisting of one thousand years.
"The ancient prophecy predicted a great transformation at the beginning of each millennium."
In plain English: A millennium is a period of time that lasts for 1,000 years.
"The new millennium began on January 1, 2001."
Usage: Use this term to describe any span of exactly 1,000 years or specifically the transition between centuries in historical contexts. It is often confused with "century," which refers only to a hundred-year period.
The word comes from the Late Latin term mīllennium, which was formed by combining the Latin adjective mīllennis meaning "lasting 1,000 years" with a suffix used to create abstract nouns. Originally, it referred specifically to a period of one thousand years.