A millennium (plural millennia or millenniums) is a period equal to 1000 years, also called kiloyears. 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). Sometimes in use, such an interval called a "millennium" might be interpreted less precisely, i.e., not always being exactly 1000 years long. It could be, for example, 1050, etc.
There are two methods of counting years: current years (the count begins at the epoch) and elapsed years (the count is of completed years since the epoch). This latter method is used in India.
The original method of counting years was ordinal, whether 1st year AD or regnal 10th year of King Henry VIII. This ordinal numbering is still in the names of the millennia and centuries, for example 1st Millennium or the 10th century, and sometimes in the names of decades, e.g., 1st decade of the 11th century.
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 100 years in common with the twentieth century, but does not include 2000, and includes 1900.
Those following ordinal year names naturally choose
Those who are influenced by the leading digit equally naturally choose
The common Western calendar (the Gregorian calendar) has been defined with counting origin 1. Thus each period of 1,000 years concludes with a year number with 3 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 viewpoint relies on the formal operation of the calendar, while the other appeals to other notions that attract popular sentiment. Stephen Jay Gould argued that the choice 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.