[RegulatorCountry]

I'll have to look around for a source, but the meaning is clear in the original Latin.

“Millennium” is singular, ergo a singularity, and so the plural of that implies many singularities, a separation and discontinuity of eras.

“Millennia” is plural in and of itself, and implies continuity, an uninterrupted era.

So, “millennia” is the term the author should have chosen, and would have, if he had any grasp of Latin.

[hecht]

Some dictionaries now list the nonsense word “millenniums” to reflect its usage by the less educated. It is akin to dictionaries now listing “loan” as a verb. “Friends, Romans , countryman LOAN me your ears”

[ApplegateRanch]

“Millennium” is singular, ergo a singularity, and so the plural of that implies many singularities, a separation and discontinuity of eras.

“Millennia” is plural in and of itself, and implies continuity, an uninterrupted era.

So, “millennia” is the term the author should have chosen, and would have, if he had any grasp of Latin.

YOU know you're right; I know you're right; but I can't find the needed source, either.

"The Third through the First Millennia BC saw less technological innovation than...,"; "In Asian history, there were three separate and distinct millenniums that saw...."