A.D. Look up A.D. at Dictionary.com
1570s, an abbreviation of Latin Anno Domini "Year of the Lord." This system of counting years was put forth by Dionysius Exiguus in 527 or 533 C.E., but used at first only for Church business. Introduced in Italy in 7c., France (partially) in 8c. In England, first found in a charter of 680 C.E. Ordained for all ecclesiastical documents in England by the Council of Chelsea, July 27, 816.

The resistance to it might have come in part because Dionysius chose 754 A.U.C. as the birth year of Jesus, while many early Christians would have thought it was 750 A.U.C. (See John J. Bond, "Handy-Book of Rules and Tables for Verifying Dates With the Christian Era," 4th ed., London: George Bell & Sons, 1889.) There is a use of simple a for anno domini in an English document from c. 1400; A.C., for Anno Christi, also was common 17c.