a central part about which other parts are grouped or gathered; core: A few faithful friends formed the nucleus of the club.
Biology. a specialized, usually spherical mass of protoplasm encased in a double membrane, and found in most living eukaryotic cells, directing their growth, metabolism, and reproduction, and functioning in the transmission of genic characters.
Physics. the positively charged mass within an atom, composed of neutrons and protons, and possessing most of the mass but occupying only a small fraction of the volume of the atom.
Anatomy. a mass of nerve cells in the brain or spinal cord in which nerve fibers form connections.