nu·cle·ar / ˈn(y)oōklēər; -kli(ə)r/ • adj. 1. of or relating to the nucleus of an atom. ∎  denoting, relating to, or powered by the energy released in nuclear fission or fusion: nuclear energy | nuclear submarines. ∎  denoting, possessing, or involving weapons using this energy: a nuclear bomb | nuclear nations.2. Biol. of or relating to the nucleus of a cell: nuclear DNA.

nuclear nuclear family a couple and their dependent children, regarded as a basic social unit.
nuclear umbrella the supposed protection gained from an alliance with a country possessing nuclear weapons.
nuclear winter a period of abnormal cold and darkness predicted to follow a nuclear war, caused by a layer of smoke and dust in the atmosphere blocking the sun's rays.