The term  (Latin for 'before Christ [was] born'), usually abbreviated to a. Chr. n., a.Ch.n., a.C.n., A.C.N., or ACN, denotes the years before the birth of Jesus Christ. It is a Latin equivalent to the English "BC" ("before Christ"). The phrase ante Christum natum is also seen shortened to ante Christum ("before Christ"), similarly abbreviated to a. Chr., A. C.  or AC. A related phrase, p. Chr. n., p. Ch. n., or post Christum natum complements a. Ch. n. and is equivalent to  (AD).

In English, these phrases are rare and AC, ACN, and ante Christum natum are not in the Chicago Manual of Style (14th edition), the American Heritage Dictionary (3rd edition), or P. Kenneth Seidelmann's Explanatory Supplement to the Astronomical Almanac (1992, University Science Books).  In other European languages, such as Italian ("a.c." or "a.C." for avanti Cristo), a vernacular version is the standard  term.

The Anglo-Saxon historian Bede used the Latin phrase  ("before the time of the Incarnation of the Lord") in his Historia ecclesiastica gentis Anglorum (Ecclesiastical History of the English People) (Book 1, Chapter 2) of 731 PCN, and thereby became the first author to describe a year as being before Christ. Both Dionysius Exiguus and Saint Bede, who was familiar with the work of the former, regarded Anno Domini 1 as beginning on the date of the Incarnation of Jesus Christ, yet "the distinction between Incarnation and Nativity was not drawn until the late 9th century, when in some places the Incarnation epoch was identified with Christ's conception, i. e., the Annunciation on March 25".

See also
 Chronology of Jesus
 Common Era
 List of ecclesiastical abbreviations
 Nativity of Jesus

References

Sources
 

Calendar eras
Latin religious words and phrases
Nativity of Jesus