A millennium (plural millennia) was a period of time equal to one thousand years.
There were differing opinions as to whether the year 2000 or 2001 was the start of the third millennium. American news outlets and world-renowned chronologist Professor Wagg considered 2000 the start of the third millennium. (TV: Doctor Who) This view was shared by at least the Eighth Doctor. (PROSE: Doctor Who - The Novel of the Film) Despite the predilection of most humans to celebrate it as the beginning of the millennium, however, at least the Sixth Doctor regarded it as the final year of the second millennium. (PROSE: Millennial Rites) This tug-of-war between pedantic and common conception of the year was reflected in an argument had between Anji Kapoor and her boyfriend Dave Young. He held fast to the technically correct view that 1 January 2001 was the beginning of the millennium, whereas she found this an example of why she was seriously considering leaving him. (PROSE: Escape Velocity)