An H-bomb is a hydrogen bomb: a nuclear weapon far more powerful than the older atomic bomb (the A-bomb). The H is for hydrogen. The bomb gets its huge energy from joining hydrogen nuclei together, which is the same kind of reaction that makes the sun shine. People say H-bomb in news, history, and everyday talk; scientists more often say hydrogen bomb or thermonuclear weapon. It is a heavy, frightening word, used about war and the Cold War. Though actually the first hydrogen bombs still needed an atomic bomb inside them to start the hydrogen reaction — so H-bomb is the popular name, not a totally separate invention.