"Ano Domini" is a Latin term meaning "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 counts years from the traditional date of the birth of Jesus Christ, marking the transition from BC (Before Christ) to AD (Anno Domini). The concept was introduced by the monk Dionysius Exiguus in the 6th century. His goal was to create a Christian calendar that would help determine the date of Easter. Over time, the use of Anno Domini spread throughout Europe and became the standard for dating historical events.