A millennium (pl. millennia) is a period of time equal to one thousand years (from Latin mille, thousand, and 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 0 or the year 1) or in later years which are whole number multiples of a thousand years after it. This concept is the one primarily discussed in this article.
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, being not necessarily exactly 1,000 years long.