Plural of Millennium
“Millennium” has two current plural forms: “millennia” and “millenniums.” Neither form needs to be treated as an error. The practical editing decision is to choose a form that fits the document and then keep that choice consistent.
Both “millennia” and “millenniums” are accepted plurals of “millennium.” Use one form consistently unless a house style or surrounding text establishes a preference.
Key details
Choose the right form
| Meaning / situation | Form / construction | Decision |
|---|---|---|
| one period of 1,000 years | millennium | Use the singular. |
| more than one | millennia | Accepted plural. |
| more than one | millenniums | Also accepted plural. |
Quick tests
Plural-choice test
Are you referring to more than one millennium?
If yes: Use either accepted plural, “millennia” or “millenniums.”
If no: Use singular “millennium.”
Caveat: A house style may prefer one form, but the reviewed dictionary evidence recognizes both.
Examples
The monument survived for two millennia.
“Millennia” is an accepted plural.
The archive spans several millenniums.
“Millenniums” is also accepted.
A millennium is one thousand years.
The singular remains “millennium.”
Further guidance
Core difference
Use either “millennia” or “millenniums” for more than one millennium.
Fast Test
If your source or house style already uses one plural, match it; otherwise either accepted form works.
Usage Boundary
Do not label the regular English plural “millenniums” incorrect merely because the Latin-derived “millennia” is common.
Sources and evidence
Sources are shown with the role they play in this guide. Historical or style-sensitive claims are kept within the evidence boundary described above.
- MILLENNIUM | English meaning - Cambridge Dictionary (opens in a new tab)Cambridge University Press · Cambridge gives both millennia and millenniums as accepted plurals of millennium.