Noun

Singular millennium

Plural millennia or millenniums

millennium (plural millennia or millenniums)

1. A period of time consisting of one thousand years.
2. (Christianity) The period of one thousand years during which Christ will reign on earth (according to some interpretations of Revelation 20:1-5).
3. A period of universal happiness, peace or prosperity; a utopia.
   • 1902, William James, The Varieties of Religious Experience, Folio Society 2008, p. 318:

     But the aggressive members of society are always tending to become bullies, robbers, and swindlers; and no one believes that sucha state of things as we now live in is the millennium.

4. (with definite article) The year 2000.

A millennium (pl. -ni⋅ums, -ni⋅a) is a period of time equal to one thousand years (from Latin mille, thousand, and annus, year) numerically to a particular dating system, specifically ones that begin at the starting (initial reference) point of the calendar in question (typically the year 1) or in later years which are whole number multiples of a thousand years after it. The term can also refer to an interval of time beginning on any date. Frequently in the latter case (and sometimes also in the former) it may have religious or theological implications (see Millenarianism). Especial

Counting years

Ordinal

The original method of counting years was ordinal, whether 1st year A.D. or regnal 10th year of King Henry VIII. This ordinal numbering is still present in the names of the millennia and centuries, for example 1st Millennium or the 20th century, and sometimes in the names of decades, e.g. 1st decade of the 21st century.

Ranges

A change from ordinals to cardinals is incomplete and might not ever be completed; the main issues arise from the content of the various year ranges. Similar issues affect the contents of centuries. Decades are usually referred to by their leading numbers and are therefore immune to this controversy: the decade called 1990s would by its naming not include 2000. Similarly the 100 years comprising the 1900s share 99 years in common with the 20th century, but do not include 2000.

Those following ordinal year names naturally choose

• 2001–2100 as the current century
• 2001–3000 as the current millennium

Those following cardinal year names equally naturally choose

• 2000–2099 as the current century
• 2000–2999 as the current millennium