/nooh"klee euhs, nyooh"-/ , n. , pl. nuclei /-klee uy'/ , nucleuses .
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.
[ 1695-1705; nuculeus, equiv. to nucu ( la ) little nut ( nuc-, s. of nux nut + -ula -ULE) + -leus n. suffix ]
Syn. 1. center, kernel, heart.