A millennium (pl. millennia) is a period of time equal to one thousand years (from Latin lang: mille, thousand, and lang: annum, year). The term may implicitly refer to calendar millennia; periods tied 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). Especially in religious usage such an interval may be interpreted less precisely, not being, exactly, 1000 years long.

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.

Cardinal

In recent years, most people have moved to expressing individual years as cardinal numbers, for example 1945 or 1998. The usage 1999th year A.D. is no longer found. This follows scientific usage, for example astronomical year numbering. As a result, some other calendar names have also moved to cardinals, e.g. 1980s is an acceptable name for a particular decade. However, 1600s could be understood as either a decade or a 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 decades and centuries.

Those following ordinal year names naturally choose

• 2001–2010 as the current decade
• 2001–2100 as the current century
• 2001–3000 as the current millennium

Those following cardinal year names equally naturally choose

• 2000–2009 as the current decade
• 2000–2099 as the current century
• 2000–2999 as the current millennium

Debate over millennium celebrations

The common Western calendar, i.e. the Gregorian calendar, has been defined with counting origin 1. Thus each period of 1,000 years concludes with a year number with three zeroes, e.g. the first thousand years in the Western calendar included the year 1000. However, there are two viewpoints about how millennia should be thought of in practice, one which relies on the formal operation of the calendar and one which appeals to other notions that attract popular sentiment.

There was a popular debate leading up to the celebrations of the year 2000 as to whether the beginning of that year should be understood (and celebrated) as the beginning of a new millennium. Historically, there has been debate around the turn of previous decades, centuries, and millennia.