What does AD stand for?

A.D. stands for the Latin phrase Anno Domini meaning Year of the Lord and it dates from the arbitratary birth of Christ.

It was introduced by the monk Dennis the Short (Dionysius Exiguus) in about 525. The date of the Incarnation was recognised by the St Bede the Venerable to be inaccurate by about five years however as it was in common use by this time the error was allowed to stand.