A nuclear weapon is an explosive device that derives its destructive force from nuclear reactions, either fission or a combination of fission and fusion.
Both reactions release vast quantities of energy from relatively small amounts of matter.
The first fission ("atomic") bomb test released the same amount of energy as approximately 20,000 tons of TNT.
The first thermonuclear ("hydrogen") bomb test released the same amount of energy as approximately 10,000,000 tons of TNT.
