1. The positively charged core of an atom about which its electrons orbit. Almost all the mass of an atom resides in its nucleus, the diameter of which is about 10⁴ times smaller than that of the atom. The nucleus of an (electrically neutral) atom is made up of protons, equal in number to its electrons, and neutrons bound together by nuclear forces. 2. In physical meteorology, a particle of any nature that initiates a phase transition in an environment supersaturated or supercooled with respect to a phase with lower chemical (Gibbs) potential, for example, a solid or liquid particle or gas/vapor bubble in a supercooled/supersaturated environment. See Aitken nucleus.