nu·cle·us (no͞oˈklē-əs)

1. A large, membrane-bound, usually spherical protoplasmic structure within a living cell, containing the cell's hereditary material and controlling its metabolism, growth, and reproduction. Also called karyon.
2. A membraneless structure in microorganisms that contains genetic material but does not itself replicate. Also called nucleoid.
3. A group of specialized nerve cells or a localized mass of gray matter in the brain or spinal cord.
4. The substance around which a urinary or other calculus forms.
5. The positively charged central region of an atom that is composed of protons and neutrons and that contains almost all of the mass of the atom.
6. A group of atoms bound in a structure, such as a benzene ring, that is resistant to alteration in chemical reactions.