What is Anno Domini?

Legal Definition
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 translated as in the year of our Lord. It is occasionally set out more fully as anno Domini nostri Iesu (or Jesu) Christi ("in the year of Our Lord Jesus Christ"). Dionysius Exiguus of Scythia Minor introduced the AD system in AD 525, counting the years since the birth of Christ.

This calendar era is based on the traditionally reckoned year of the conception or birth of Jesus of Nazareth, with 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, 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.

The Gregorian calendar is the most widely used calendar in the world today. For decades, it has been the unofficial global standard, adopted in the pragmatic interests of international communication, transportation, and commercial integration, and recognized by international institutions such as the United Nations.

Traditionally, English followed Latin usage by placing the "AD" abbreviation before the year number. However, BC is placed after the year number (for example: AD 2017, but 68 BC), which also preserves syntactic order. 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 approximate 33 years commonly associated with the life of Jesus would not be included in either of the BC and 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.
-- Wikipedia
Legal Definition
In the year of our Lord, abbreviated, A. D. The computation of time from the incarnation of our Saviour which is used as the date of all public deeds in the United tites and Christian countries, on which account it is called the "vulgar vera."
-- Bouviers Law Dictionary
Legal Definition
In the year of the Lord. Commonly abbreviated A. D. The computation of time, according to the Christian era, dates from the birth of Christ. This phrase has become Anglicized by adoption, so that an indictment or declaration containing the words "Anno Domini" is not demurrable as not being in the English language. State v. Gilbert, 13 Vt. 647; Halev. Vesper, Smith (N. Hil 283.
-- Black's Law Dictionary
Legal Definition
In the year of our Lord; since the birth of Christ.
-- Ballentine's Law Dictionary