B.C. is an abbreviation for "Before Christ," used to indicate that one is speaking about the historical period preceding the estimated year of the birth of Jesus. The abbreviation A.D. may be used to refer to the period following the estimated year of the birth of Jesus.
A.D. stands for "anno Domini," the Latin term meaning "in the year of our Lord."
In recent years, the abbreviation B.C.E., or "before the Common Era," has gained in popularity. B.C.E. is used interchangeably with B.C., but without making reference to any religion or religious figure. Similarly, C.E., or "Common Era," may be used in place of A.D. to maintain a secular tone.