A millennium is a time period that spans one thousand years or the one-thousand-year anniversary of something. The plural of millennium may be rendered as millennia or as millenniums. Millennium is a Modern Latin word, derived from the Latin word mille which means thousand and the Latin word annus, which means year. As a Latin word, the plural of millennium is rendered as millennia. However, since millennium is now an appropriated English word, it is also correct to render the plural form as millenniums. According to Google’s Ngram, the plural form millenniums was most popular until the mid-1930s, today the plural form millennia is far more popular.
Sea Levels Are Rising More Quickly Than in the Last Two Millennia (The Smithsonian Magazine)
The fossils were trapped and preserved for millennia in sediment along the banks of an ancient rivers. (Discover Magazine)
The songs on “Steady Now” are sweeping and majestic, built around intricate layered harmonies and instrumental flourishes that expand Sale’s sonic palate — steel guitar notes that linger like the moan of wind through cracks in old barn boards, piano that climbs over gentle percussion like the waters of a mountain stream colliding with millennia-old rocks, bass licks that lope around the edges like a bobcat on the edge of a campfire’s glow. (The Daily Times)
“The introduction of cannabis price data into our MarketView® product suite is unprecedented and demonstrates that cannabis is becoming a viable alternative for traditional commercial farmers facing the millennia-old question of crop switching and rotation,” said Charles Trauger, Global Director of Agriculture at GlobalView. (Forbes Magazine)
For nearly three millenniums, the Earth’s ocean levels fluctuated by just three inches, rising no more than one or one and a half inches per century. (The Christian Science Monitor)
Such tombs, built by the Funnel Beaker Culture between the fifth and third millenniums B.C., are generally triangular in shape and surrounded with stone blocks. (Archaeology Magazine)