1. a period of 1,000 years
2. a period of great happiness, prosperity, or achievement

Examples

• The new millennium began on January 1, 2001.
• Scientists study climate changes over millennia.
• The ancient civilization flourished for more than a millennium.
• Many people celebrated the turn of the millennium with fireworks.
• The discovery promised to usher in a new millennium of medical advances.
• Archaeologists found artifacts dating back several millennia.
• The millennium bug caused widespread concern about computer systems.