A millennium is 1,000 years.

To give a bit more feel for that span of time:

  • 1 millennium = 10 centuries (10 × 100 years).
  • 1 millennium = 100 decades (100 × 10 years).
  • In the Gregorian calendar, the 1st millennium ran from year 1 to 1000, the 2nd from 1001 to 2000.

So when people talk about “the new millennium,” they mean the start of a new 1,000‑year block on the calendar.