1) A millennium is a period of one thousand years. It is similar to the terms biennium , a period of two years, and century , a period of one hundred years. The term derives from the Latin mille , meaning thousand, and annum , meaning year.
2) The millennium is the anniversary or celebration of a 1000-year period. In the United States, the official timekeeper at the Naval Observatory considers that the second millennium and the beginning of the third will be reached on January 1, 2001. This date is based on the Gregorian calendar , created in 1582 Anno Domini (A.D.) and which has since become a world standard for civil affairs. The Gregorian calendar uses the table of dates for Easter that was established by the sixth-century scholar Dionysius Exiguus who marked the modern epoch as beginning on January 1, 1 A.D. For this reason, the second millennium is not actually reached until January 1, 2001. However, because the most dramatic change in the calendar occurred on January 1, 2000, much of the world celebrated the beginning of the third millennium a year early.
3) The millennium is also used to refer in general to a time when great achievements finally come to pass, great happiness prevails, or some other important objective is reached. In the Book of Revelations, the millennium is a period of a thousand years during which Christ is to rule on Earth.