Definition of B.C. abbreviation from the Oxford Advanced American Dictionary

B.C.

abbreviation (also BC)
/ˌbi ˈsi/
 
 
before Christ (used in the Christian calendar to show a particular number of years before the year when Christ is believed to have been born)
in (the year) 2,000 B.C.the third century B.C. compare A.D., A.H., B.C.E., C.E.