A millennium (plural millennia or millenniums) is a period of time 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.