A millennium (plural millennia or millenniums) is a period equal to 1000 years,[1] sometimes called a kiloyear. It derives from the Latin mille, thousand, and annus, year. It is often, but not always, related to a particular dating system.

Sometimes, it is used specifically for periods of a thousand years that begin at the starting point (initial reference point) of the calendar in consideration (typically the year "1"), or in later years that 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).

Debate over millennium celebrations

There was widespread public 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 "the" new millennium. Historically, there has been debate around the turn of previous decades, centuries, and millennia. The issue arises from the difference between the convention of using ordinal numbers to count years and millennia (as in "the third millennium"), or cardinally using "the two thousands". The first convention is common in English-speaking countries, but the latter is favoured in, for example, Sweden (tvåtusentalet, which translates literally as the two thousands period). Those holding that the arrival of the new millennium should be celebrated in the transition from 2000 to 2001 (i.e., December 31, 2000 to January 1, 2001) argued that the Gregorian calendar has no year zero, and therefore the millennia should be counted from AD 1. Thus, the first period of one thousand complete years runs from the beginning of AD 1 to the end of AD 1000, and the beginning of the second millennium took place at the beginning of 1001. The "year 2000" has been a popular phrase referring to an often utopian future, or a year when stories in such a future were set, adding to its cultural significance. There was also media and public interest in the Y2K computer bug. The change from 1999 to 2000 was compared to the "rolling over" of zeroes on an odometer. Some people[2] argued that the change of the hundreds digit in the year number, and the zeroes rolling over, created a sense that a new century had begun. This is analogous to the common demarcation of decades by their most significant digits, e.g., naming the period 1980 to 1989 as the 1980s or "the eighties". The ISO 8601 standard is superficially similar to this second viewpoint, but as it includes a "year 0" it is not strictly relevant to the discussion.

Stephen Jay Gould argued that the choice between them is arbitrary, and, since the question revolves around rules made by people, rather than a natural phenomenon that is subject to experimental measurement, the matter cannot be resolved.[3] Gould, in his essay Dousing Diminutive Dennis' Debate (or DDDD = 2000) (Dinosaur in a Haystack), discussed the "high" versus "pop" culture interpretation of the transition. Gould noted that the high culture, strict construction had been the dominant viewpoint at the 20th century's beginning, but that the pop culture viewpoint dominated at its end. Gould also included comments on adjustments to the calendar, such as those by Dionysius Exiguus (the eponymous "Diminutive Dennis") and the timing of celebrations over different transitional periods.

The popular[4] approach was to treat the end of 1999 as the end of "a millennium" and to hold millennium celebrations at midnight between December 31, 1999, and January 1, 2000, as per viewpoint 2. The cultural and psychological significance of the events listed above combined to cause celebrations to be observed one year earlier than the formal Gregorian date.[4] However, this does not establish that insistence on the formal Gregorian date is "incorrect". Some event organisers hedged their bets by calling their 1999 celebrations things like "Click" referring to the odometer-like rolling over of the nines to zeroes; another, pragmatic approach was to celebrate "a new millennium" twice.

Viewpoint 1: Strict usage

2 BC 1 BC AD 1 2 3 4 5 ... 998 999 1000 1001 1002 1003 ... 1998 1999 2000 2001 2002 2003 ... 2998 2999 3000 3001 3002 3003 ...
First one thousand years (millennium) Second millennium Third millennium Fourth millennium

Viewpoint 2: General usage

Illustration of years with a 99–00 demarcation (starting AD 1)
1 BC AD 1  2   3   4   5   ...  998 999 1000 1001 1002 ... 1998 1999 2000 2001 2002 ... 2998 2999 3000 3001 3002
First millennium Second millennium Third millennium Fourth millennium
Other Languages
Afrikaans: Millennium
Alemannisch: Jahrtausend
العربية: ألفية
asturianu: Mileniu
Avañe'ẽ: Su ary
azərbaycanca: Minillik
Bân-lâm-gú: Chhian-liân-kí
башҡортса: Мең йыллыҡ
беларуская: Тысячагоддзе
беларуская (тарашкевіца)‎: Тысячагодзьдзе
български: Хилядолетие
bosanski: Milenij
català: Mil·lenni
čeština: Tisíciletí
Cymraeg: Mileniwm
dansk: Millennium
Deutsch: Jahrtausend
eesti: Aastatuhat
Ελληνικά: Χιλιετία
emiliàn e rumagnòl: Milèni
español: Milenio
Esperanto: Jarmilo
euskara: Milurteko
فارسی: هزاره
français: Millénaire
Frysk: Milennium
Gàidhlig: Mìle-bliadhna
galego: Milenio
ГӀалгӀай: Эзар шу
客家語/Hak-kâ-ngî: Chhiên-ki
한국어: 밀레니엄
հայերեն: Հազարամյակ
hrvatski: Tisućljeće
Ilokano: Milenio
Bahasa Indonesia: Milenium
interlingua: Millennio
íslenska: Árþúsund
italiano: Millennio
עברית: מילניום
Jawa: Milenium
Kabɩyɛ: Pɩnzɩ kutoku
ქართული: ათასწლეული
қазақша: Миллениум
Kiswahili: Milenia
Kreyòl ayisyen: Milenè
Latina: Millennium
latviešu: Tūkstošgade
lietuvių: Tūkstantmetis
Limburgs: Millennium
la .lojban.: ki'orna'a
magyar: Millennium
македонски: Милениум
Malagasy: Taonarivo
मराठी: सहस्रक
Bahasa Melayu: Alaf
Mìng-dĕ̤ng-ngṳ̄: Chiĕng-gī
Mirandés: Milénio
монгол: Мянган
မြန်မာဘာသာ: ထောင်စုနှစ်
Nederlands: Millennium
नेपाली: सहश्राब्दि
日本語: ミレニアム
norsk: Millennium
norsk nynorsk: Millennium
Nouormand: Millénaire
ਪੰਜਾਬੀ: ਮਿਲੈਨੀਅਮ
پښتو: زرۍ
polski: Tysiąclecie
português: Milênio
Ripoarisch: Joohrdusend
română: Mileniu
Runa Simi: Waranqawata
русский: Тысячелетие
Scots: Millennium
Sesotho sa Leboa: Ngwagakete
Simple English: Millennium
سنڌي: ھزارو
slovenčina: Tisícročie
slovenščina: Tisočletje
српски / srpski: Миленијум
srpskohrvatski / српскохрватски: Milenijum
Basa Sunda: Milénium
suomi: Vuosituhat
svenska: Millennium
Tagalog: Milenyo
татарча/tatarça: Меңьеллык
Türkçe: Milenyum
Türkmençe: Müňýyllyk
українська: Тисячоліття
Vahcuengh: Cien'geij
Tiếng Việt: Thiên niên kỷ
Winaray: Yukottuig
吴语: 千纪
Yorùbá: Ẹ̀rúndún
粵語: 千年
Zeêuws: Millennium
žemaitėška: Tūkstontmetis
中文: 千纪