The term ante Christum natum (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. It is a Latin equivalent to the English term "BC" ("before Christ"). The phrase ante Christum natum is also seen shortened to ante Christum (Latin for "before Christ"), similarly abbreviated to a. Chr., A.C. or AC. A related term, p. Chr. n., p.Ch.n, or post Christum natum complements a.Ch.n and is equivalent to "AD".
English speakers are unlikely to recognize these terms, which are so rare that AC, ACN, and ante Christum natum are not in the Chicago Manual of Style (14th ed.), the American Heritage Dictionary (3rd ed.), or P. Kenneth Seidelmann's Explanatory Supplement to the Astronomical Almanac (1992, University Science Books).
The Venerable Bede, who was the first writer to identify a year as before Christ, used the Latin ante incarnationis dominicae tempus (before the time of the incarnation of the Lord) in his Historia ecclesiastica gentis Anglorum (I.2) in 731.