How many years make up a millennium?

A millennium is a period of 1,000 years.

The word comes from Latin: mille (thousand) + annus (year).

Examples:

The 2nd millennium ran from January 1, 1001 to December 31, 2000 (1,000 years).

The current 3rd millennium began January 1, 2001 and will end December 31, 3000 (again, 1,000 years).

So, straightforward answer: 1,000 years = 1 millennium.