Pronunciation: (nOO'klē-us, nyOO'-),[key] —n., —pl.-cle•iPronunciation: (-klē-ī"),[key]-cle•us•es. 1. a central part about which other parts are grouped or gathered; core: A few faithful friends formed the nucleus of the club. 2. Biol.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. See illus. under ameba. 3. 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.
4. Anat.a mass of nerve cells in the brain or spinal cord in which nerve fibers form connections.
5. Also called condensation nucleus.Meteorol.a particle upon which condensation of water vapor occurs to form water drops or ice crystals.
6. Chem.a fundamental arrangement of atoms, as the benzene ring, that may occur in many compounds by substitution of atoms without a change in structure.
7. Astron.the condensed portion of the head of a comet.
8. Phonet. a. the central, most prominent segment in a syllable, consisting of a vowel, diphthong, or vowellike consonant, as the a-sound in cat or the l-sound in bottled; peak.
b. the most prominent syllable in an utterance or stress group; tonic syllable.