“Anno domini: Latin for ‘in the Year of Our Lord’; i.e., in the year since the nativity: generally abbreviated to AD. It was Dionysus Exiguus who fixed the date of the Nativity; he lived in the early 6th century, and his computation is probably late by some three to six years. The custom of determining dates on this basis is said to be the result of the work of the Venerable Bede.”
Excerpted from: Murphy, Bruce, ed. Benet’s Reader’s Encyclopedia, Fourth Edition. New York: Harper Collins, 1996.