B.C. and A.D. are short for "Before Christ" and "Anno Domini," respectively. The latter is a shortened form of the Medieval Latin phrase "Anno Domini Nostri Iesu Christi," which translates as "In The Year of Our Lord Jesus Christ."
B.C. and A.D. are used to mark years prior to or after the epochal, or origin event of the era, namely the birth of Jesus of Nazareth. The system was designed in 525 by Dionysus Exiguus as a replacement for the Diocletian designation system and became popular throughout Europe from the 11th through 14th centuries.
Neutral alternatives to the Christian-centric designations include the abbreviations B.C.E. and C.E., which stand for "Before Common Era" and "Common Era."