Nuclear means relating to the nucleus, which is the central core of something. In science, it most often refers to the nucleus of an atom, the incredibly tiny center where nearly all of an atom's mass is concentrated.

Nuclear energy comes from splitting apart or fusing together atomic nuclei, releasing enormous amounts of power. A single uranium pellet the size of a fingertip contains as much energy as a ton of coal. Nuclear power plants use this energy to generate electricity for millions of homes. Nuclear weapons use the same energy destructively, which is why nuclear war became one of humanity's greatest fears during the Cold War between the United States and the Soviet Union.

When something goes nuclear, it means a situation has escalated dramatically, like when a small argument suddenly explodes into a huge fight. This meaning comes from how nuclear reactions release massive amounts of energy very quickly.

The word also appears in nuclear family, meaning parents and their children living together as one household, suggesting that the family is the core unit of society.