An A-bomb is an atomic bomb: a hugely powerful bomb whose energy comes from splitting atoms, the tiny pieces that make up matter. The name is simply a short form of “atomic bomb.” Newspapers and older books used A-bomb a great deal in the twentieth century, especially after 1945. Today people more often say “nuclear bomb” or “nuclear weapon,” but A-bomb is still understood. It is a heavy, serious word; we do not use it as a joke or for something merely “very strong.” There is a parallel short name, H-bomb, for a hydrogen bomb, which works in a different way and is even more powerful. You will meet A-bomb in history writing more than in everyday talk.