What is the scientific meaning of AD and BC?

Ad means after Christ died And BC means befor christ.

Elizabeth L. AD after Jesus' death BC Before Christ A.D. means anno domini. It is latin for 'In the year of The Lord', or 'In the year of Our Lord'. It is usually understood to be year 1 in the Gregorian calander. B.C. means before common, current or christian era. This has recently been changed to B.C.E. for before common era. See the related link for more information.