A thermonuclear weapon is a nuclear weapon that uses the energy from a primary nuclear fission reaction to compress and ignite a secondary nuclear fusion reaction. The result is greatly increased explosive power when compared to single-stage fission weapons. It is colloquially referred to as a hydrogen bomb or H-bomb because it employs fusion of isotopes of hydrogen.

The first full scale thermonuclear test was carried out by the United States in 1952; the concept has since been employed by most of the world's nuclear powers in the design of their weapons. The modern design of all thermonuclear weapons in the United States is known as the Teller–Ulam configuration for its two chief contributors, Edward Teller and Stanislaw Ulam, who developed it in 1951 for the United States, with certain concepts developed with the contribution of John von Neumann. Similar devices were developed by the Soviet Union, United Kingdom, China, and France.