Nucleus (noo’kle-us) pl. nuclei 1. The general oval protoplasmic body in the center of the cell that contains the chromosomes and is surrounded by a membrane; an essential organelle that controls metabolism, growth, and reproduction. 2. A localized mass of gray matter within the brain and spinal cord, composed of nerve cells. 3. The heavy, central, positively charged portion of the atom (composed of protons and neutrons); it constitutes the mass of the atom, about which the electrons revolve in orbit. 4. A central part or mass.