Anno Domini
Anno Domini (AD) and before Christ (BC) qualify years in the Gregorian and Julian calendars, whose epoch is the traditional year of the conception or birth of Jesus. AD counts years since the epoch, BC the years before the epoch.
Anno Domini is Medieval Latin for "in the year of the Lord", often read as "our Lord". "BC" is specific to English, and equivalent abbreviations are used in other languages: the Latin form, rarely used in English, is ante Christum natum (ACN) or ante Christum (AC).
This calendar era takes as its epoch the traditionally reckoned year of the conception or birth of Jesus. Years AD are counted forward since that epoch and years BC are counted backward from the epoch. There is no year zero; the year AD 1 immediately follows the year 1 BC. The system was devised in 525, in Rome, by the Eastern Roman monk Dionysius Exiguus, but was not widely used until the 9th century. Modern scholars believe that the actual date of the birth of Jesus was about 5 BC.
To avoid religious associations, many writers and scholars prefer CE (Common Era) and BCE (Before the Common Era) instead of AD and BC.
Astronomical year numbering and ISO 8601 do not use words or abbreviations related to Christianity, but use the same numbers for AD years (but not for BC years since the astronomical year 0 is 1 BC).