Other forms: millennia
A millennium is a thousand years. Two millennia stand between us and Rome, and the last one turned over at midnight as 1999 became 2000, with fireworks in every capital.
The plural is millennia, and the spelling is one of the traps of English: two Ls and two Ns, because the Latin words for thousand and for year are welded together in the middle. Millennial is the adjective, and with a capital letter it is also a person, someone who grew up around that turn of the century. The Millennium Bug was the fear that computers storing the year as two digits would read 00 as 1900 and fall over. In the Book of Revelation the millennium is a thousand-year reign of peace, and millenarian movements take their name from it. Beside it sit decade for ten years and century for a hundred.
1nounperiodA period of one thousand years.
“The stones have stood on that hill for two millennia.”
2nounmomentThe moment when one thousand-year period ends and another begins, especially the year 2000.
“They were in Sydney for the millennium.”
3nounmillennium(in religion) a coming age of a thousand years of peace.
“The sect preached that the millennium was at hand.”
millennium is a length of time.
From Latin millennium, from millennis, meaning 1000-year. The awkward double letters in the middle are the join between the two Latin words behind it, one for thousand and one for year, which is exactly where people go wrong when they spell it.
Latin
mīllennium
English, today
millennium
Borrowed from Latin mīllennium. From Latin mīllennium, from Latin mīllennis (“1000-year”) + -ium.
Etymology from Wiktionary, CC BY-SA
Practised in Time & Calendar, History & Eras