The second millennium was a period of time that began on January 1, 1001 of the Julian calendar and ended on December 31, 2000 of the Gregorian calendar. It was the second period of one thousand years in the Anno Domini or Common Era. It is distinct from the millennium known as the 1000s which began on January 1, 1000 and ended on December 31, 1999.
It encompassed the High and Late Middle Ages, the Mongol Empire, the Renaissance, the Baroque era, the early Modern Age, the age of Enlightenment, the age of colonialism, industrialization, the rise of nation states, and the 19th and 20th century with the impact of science, widespread education, and universal health care and vaccinations in many nations. The centuries of expanding large-scale warfare with high-tech weaponry were offset by growing peace movements, the United Nations, plus doctors and health workers crossing borders to treat injuries and disease, and the return of the Olympics as contest without combat.