nuclear. adj. 1. (General Physics) of, concerned with, or involving the nucleus of an atom: nuclear fission. 2. (Biology) biology of, relating to, or contained within the nucleus of a cell: a nuclear membrane. 3. of, relating to, forming, or resembling any other kind of nucleus. 4. (General Physics) of, concerned with, or operated by energy from fission or fusion of atomic nuclei: a nuclear weapon.