Merriam-Webster Collegiate Dictionary - nucleus

 noun  (plural nuclei; also ~es)  Etymology: New Latin, from Latin, kernel, from nuc-, nux nut — more at nut  Date: 1704  1.  a. the small bright body in the head of a comet  b. the small brighter and denser portion of a galaxy  2. a central point, group, or mass about which gathering, concentration, or accretion takes place: as  a. a cellular organelle of eukaryotes that is essential to cell functions (as reproduction and protein synthesis), is composed of nucleoplasm and a nucleoprotein-rich network from which chromosomes and nucleoli arise, and is enclosed in a definite membrane — see cell illustration  b. a mass of gray matter or group of cell bodies of neurons in the central nervous system  c. a characteristic and stable complex of atoms or groups in a molecule; especially ring the naphthalene ~  d. the positively charged central portion of an atom that comprises nearly all of the atomic mass and that consists of protons and usually neutrons  3. the peak of sonority in the utterance of a syllable  4. a basic or essential part ; core players who are the ~ of the team