A millennium (plural millennia or millenniums) is a period of one thousand years, sometimes called a kiloyear. Sometimes, the word 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") and at later years that are whole number multiples of a thousand years after the start point. The term can also refer to an interval of time beginning on any date. Millennia sometimes have religious or theological implications (see millenarianism).
The word millennium derives from the Latin mille, thousand, and annus, year.
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 the year 1. Thus, the first millennium was from the year 1 to the end of the year 1000, the second millennium from 1001 to the end of 2000, and so on. 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 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 'tens' digit, 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 ISO 8601 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. 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 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, with the cultural and psychological significance of the events listed above combining to cause celebrations to be observed one year earlier than the formal Gregorian date. 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.