The full form of “AD” is “Anno Domini,” which is a Latin term meaning “in the year of our Lord.” It is used to denote years in the Gregorian calendar that are counted from the traditional date of the birth of Jesus Christ.
The full form of “BC” is “Before Christ,” which is used to signify the years before the birth of Jesus Christ.
Therefore, AD refers to years after Christ’s birth, while BC refers to years before it.