A.D. is the abbreviation for anno Domini, which is Latin for in the year of our Lord. In 532, the monk Dionysius Exiguus introduced a system of dating events, beginning with the year he believed Jesus Christ was born. In this system, the year of Christ's birth was A.D. 1, and the year before that was 1 B.C. (before Christ). Modern scholars believe Christ was actually born no later than 1 B.C. But people still determine dates using the original system.