millennium

Concise Oxford English Dictionary © 2008 Oxford University Press:
millennium /mɪˈlɛnɪəm/

▶noun (pl. millennia /-nɪə/ or millenniums)

• 1 a period of a thousand years, especially when calculated from the traditional date of the birth of Christ.

  ■ (the millennium) Christian Theology the prophesied thousand-year reign of Christ at the end of the age (Rev. 20:1–5).

  ■ a utopian period.

• 2 an anniversary of a thousand years.

• 3 (the millennium) the point at which one period of a thousand years ends and another begins.

– derivatives
millennial adjective.

– origin C17: mod. L., from L. mille ‘thousand’, on the pattern of biennium.

usage: The correct spelling is millennium, with two ns. The spelling with one n is a common error, formed by analogy with other similar words correctly spelled with only one n, such as millenarian.