say it: muh-LEN-ee-uhm
millennium is a noun meaning "a span of 1000 years". It carries 3 recorded senses.
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
New Testament
the 1000th anniversary (or the celebration of it)
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for millennium come from Princeton University’s WordNet. The pronunciation comes from the CMU Pronouncing Dictionary. Commonness is measured against a corpus of everyday English. Full sources and licences.