Anno Domini, A.D. the year of Our Lord, in Latin. The Christian era began Jan. 1, in the middle of the fourth year of the 194th Olympiad, the 753rd year of the building of Rome, and in 4714 of the Julian period. This era was invented by a monk, Dionysius Exiguus, about 532. It was introduced into Italy in the 6th century, and ordered to be used by bishops by the Council of Chelsea, in 816, but was not generally employed for several centuries. Charles III. of Germany was the first who added "in the year of Our Lord" to his reign, in 879.
Entry from Everybody's Cyclopedia, 1912.