Noun

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.

Derived terms

• millennial
• millennium bug

Danish

A millennium (pl. -ni⋅ums, -ni⋅a) is a period of time equal to one thousand years (from the Latin phrase mille, thousand, and annus, year), often but not necessarily related numerically to a particular dating system.

For example, a millennium could start at the beginning of the year 289 and finish at the beginning of the year 1289.

Sometimes, it is used specifically for periods of one thousand years that begin at the starting point (initial reference point) of the calendar in consideration (Thus is 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). Sometimes in religious usages, such an interval called a "millennium" might be interpreted less precisely, i.e., not always being exactly 1000 years long.

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.