The initials A.D. (used with or without periods) is an abbreviation for the Latin "Anno Domini", which translates to "the Year of Our Lord," referring to years after the birth of Jesus Christ. The idea to break history down in such a manner is at least old as the 6th century AD, when Victor of Tunnena established a chronology of the foundation of Rome and used what is now recognized as an incorrectly calculated birth date for Christ.
For better or worse, AD/BC is the modern calendar that most of the west (in fact, most of the world), uses. Because the term is specific to the Christian religion, however, some scholars prefer to use "C.E." instead, which means "the Common Era."
Other Common Calendar Designations
Taylor, Timothy 2008 Pehistory vs. Archaeology: Terms of Engagement. Journal of World Prehistory 2:11–18.
This glossary entry is part of the About.com Guide to Calendar Designations and the Dictionary of Archaeology.
Also Known As: CE