Learn how to say Millennial correctly in English with this tutorial pronunciation video.
Oxford dictionary definition of the word millennium:
noun (plural millennia /-nɪə/ or millenniums)
1a period of a thousand years, especially when calculated from the traditional date of the birth of Christ:
silver first came into use on a substantial scale during the 3rd millennium bc
(the millennium) Christian Theology the prophesied thousand-year reign of Christ at the end of the age (Rev. 20:1-5):
Archer’s treatise predicted that the millennium was close at hand
(the millennium) a utopian period of justice, peace, and prosperity:
the people must seize power—the millennium cannot be imposed on them from above
2an anniversary of a thousand years:
the millennium of the Russian Orthodox Church
(the millennium) the point at which one period of a thousand years ends and another begins:
as we approach the millennium the only certainty is change
mid 17th century: modern Latin, from Latin mille ‘thousand’, on the pattern of biennium
The correct spelling is millennium not millenium. The latter is a common error, formed by analogy with other similar words correctly spelled with only one n, such as millenarian and millenary. The differences in spelling are explained by different origins. Millennium was formed by analogy with words like biennium, while millenary and millenarian were formed from the Latin milleni.