"Anno Domini" (often abbreviated as AD) is a Latin term that means "in the year of our Lord." It is used to label years in the Gregorian calendar, which is the calendar system most widely used today. The system was devised in the 6th century by a monk named Dionysius Exiguus, who aimed to count years from the birth of Jesus Christ. The Anno Domini system divides history into two main eras: AD for years after the birth of Christ and BC (Before Christ) for years before that event. This method of dating has become a standard way to reference historical events and timelines, helping to organize history in a clear and consistent manner.