What do "BC" and "AD" mean in a timeline?

"B.C." stands for "before Christ," while "A.D." stand for "anno domini," meaning "in the year of our Lord," according to How Stuff Works. This way of delineating the calendar was established in the 5th century.

The calendar runs from the year Jesus Christ was born, which is 1 A.D. The calendar counts backwards from the year 1 B.C. on the B.C. side and forwards on the A.D. side. In other words, the calendar runs 3 B.C., 2 B.C., 1 B.C., 1 A.D., 2 A.D., and so on. Because this method of dividing is Christian-centric, many historians and timelines use "B.C.E." and "C.E." instead. "B.C.E." stands for "before the common era," while "C.E." stands for the "common era."