The terms anno Domini (AD) and before Christ (BC) are used to label or number years in the Julian and Gregorian calendars. The term anno Domini is Medieval Latin and means 'in the year of the Lord', but is often presented using "our Lord" instead of "the Lord", taken from the full original phrase "anno Domini nostri Jesu Christi", which translates to 'in the year of our Lord Jesus Christ'. The form "BC" is specific to English and equivalent abbreviations are used in other languages: the Latin form is Ante Christum natum but is rarely seen.

This calendar era is based on the traditionally reckoned year of the conception or birth of Jesus, AD counting years from the start of this epoch and BC denoting years before the start of the era. There is no year zero in this scheme; thus the year AD 1 immediately follows the year 1 BC. This dating system was devised in 525 by Dionysius Exiguus, but was not widely used until the 9th century.

Traditionally, English follows Latin usage by placing the "AD" abbreviation before the year number, though it is also found after the year. In contrast, BC is always placed after the year number (for example: AD 70, but 70 BC), which preserves syntactic order. The abbreviation AD 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, which would mean that the approximate 33 years commonly associated with the life of Jesus would be included in neither the BC nor the AD time scales.

Terminology that is viewed by some as being more neutral and inclusive of non-Christian people is to call this the Current or Common Era (abbreviated as CE), with the preceding years referred to as Before the Common or Current Era (BCE). Astronomical year numbering and ISO 8601 avoid words or abbreviations related to Christianity, but use the same numbers for AD years (but not for BC years in the case of astronomical years; e.g., 1 BC is year 0, 45 BC is year −44).