a central part about which other parts are grouped or gathered; core

A few faithful friends formed the nucleus of the club.

Biology . 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.

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.

Anatomy . a mass of nerve cells in the brain or spinal cord in which nerve fibers form connections.

Also called condensation nucleus. Meteorology . a particle upon which condensation of water vapor occurs to form water drops or ice crystals.

Chemistry . a fundamental arrangement of atoms, as the benzene ring, that may occur in many compounds by substitution of atoms without a change in structure.

Astronomy . the condensed portion of the head of a comet.

Phonetics .

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.

the most prominent syllable in an utterance or stress group; tonic syllable.

noun

exterior , exteriority , outside

noun

beginning bud , center , crux , embryo , focus , foundation , germ , heart , hub , kernel , matter , nub , pivot , premise , principle , seed , spark , bud , cadre , core , ring