Anno Domini (AD or A.D.) and Before Christ (BC or B.C.) are designations used to label or number years used with the Julian and Gregorian calendars. This calendar era is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth, with AD counting years after the start of this epoch, and BC denoting years before the start of the epoch. There is no year zero in this scheme, so the year AD 1 immediately follows the year 1 BC. This dating system was devised in 525, but was not widely used until after 800.
Alternative names for this era include vulgaris aerae (found 1615 in Latin), "Vulgar Era" (in English, as early as 1635), "Christian Era" (in English, in 1652), "Common Era" (in English, 1708), and "Current Era". Since 1856, the alternative abbreviations CE and BCE are sometimes used in place of AD and BC.
The Gregorian calendar is the most widely used calendar in the world today. For decades, it has been the unofficial global standard, adopted for pragmatic interests of international communication, transportation and commercial integration and recognized by international institutions such as the United Nations and the Universal Postal Union.
The term Anno Domini is Medieval Latin, translated as In the year of the Lord, and as in the year of our Lord. It is sometimes specified more fully as Anno Domini Nostri Iesu (Jesu) Christi ("In the Year of Our Lord Jesus Christ").
Traditionally, English has copied Latin usage by placing the abbreviation before the year number for AD. Since BC is not derived from Latin it is placed after the year number (for example: AD 2012, but 68 BC). However, placing the AD after the year number (as in "2012 AD") is also becoming common usage. The abbreviation is also widely used after the number of a century or millennium, as in "fourth century AD" or "second millennium AD" (although conservative usage formerly rejected such expressions). Because BC is the English abbreviation for Before Christ, it is sometimes incorrectly concluded that AD means After Death, i.e., after the death of Jesus. However this would mean that the ~33 years commonly associated with the life of Jesus would not be present in either BC or AD time scales.
Some archaeologists use the lowercase letters bc and ad and bp as terminology for uncalibrated radiocarbon dates for these eras.
Read more about Anno Domini: History, Historical Birth Date of Jesus, Other Eras, Common Era, No Year Zero, Proposed Reforms