A millennium is a period of one thousand years. The word comes from Latin: mille means thousand and annum means year.

When historians divide up time, they often think in millennia (the plural of millennium). The Great Pyramid of Giza has stood for more than four millennia. When scientists study evolution or geological formations, they often measure in millennia because many natural processes unfold over these vast timescales.

You've probably heard about “the new millennium,” which refers to the period beginning in the year 2000. People celebrated this turn of the millennium with special excitement because it only happens once every thousand years. Some computer programmers worried about the “Y2K bug” because older computers might malfunction when the year changed from 1999 to 2000.

A millennium is much longer than a century (100 years) or a decade (10 years). To put it in perspective: if you're ten years old, you'd need to live one hundred lifetimes to experience a full millennium. That's why when we talk about millennia, we're usually discussing history, archaeology, or very long-term predictions about the future.