Anno Domine (Latin): "In the year of our Lord".
A method of marking time using the number of years since the birth of Jesus Christ. It is often abbreviated as "A.D."
The years before the birth of Christ are designated "Before Christ", or "B.C."
Interestingly, due to the inaccuracies of historical record keeping from that period, Christ is now thought to have been born in the year 2 C.E. by our calendar.
Recognizing the fact that not all people in the world are Christian, most archaeologists and historians now use C.E. ("Of the Common Era"), and B.C.E. ("Before the Common Era").