Millennium/mɪˈleniəm/ Definition 1 of 3Pro Tip 1/3Counting Millennia When referencing centuries, 'millennium' helps mark time in larger blocks. The discovery of fire changed human life in the first millennium. Pro Tip 2/3Historical Events Use 'millennium' to contextualize significant historical periods or events. The Roman Empire fell before the second millennium began. Pro Tip 3/3Future Projections Futuristic discussions often mention 'millennium' to frame long-term speculations. By the next millennium, humans might live on Mars.