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[noo-klee-uh s, nyoo-] /ˈnu kli əs, ˈnyu-/

noun, plural nuclei
[noo-klee-ahy, nyoo-] /ˈnu kliˌaɪ, ˈnyu-/ (Show IPA), 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.
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.
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.
Anatomy. a mass of nerve cells in the brain or spinal cord in which nerve fibers form connections.
5.
Also called condensation nucleus. Meteorology. a particle upon which condensation of water vapor occurs to form water drops or ice crystals.
6.
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.
7.
Astronomy. the condensed portion of the head of a comet.
8.
Phonetics.

/ˈnjuːklɪəs/
noun (pl) -clei (-klɪˌaɪ), -cleuses
1.
a central or fundamental part or thing around which others are grouped; core
2.
a centre of growth or development; basis; kernel: the nucleus of an idea
3.
(biology) (in the cells of eukaryotes) a large compartment, bounded by a double membrane, that contains the chromosomes and associated molecules and controls the characteristics and growth of the cell
4.
(anatomy) any of various groups of nerve cells in the central nervous system
5.
(astronomy) the central portion in the head of a comet, consisting of small solid particles of ice and frozen gases, which vaporize on approaching the sun to form the coma and tail
6.
(physics) the positively charged dense region at the centre of an atom, composed of protons and neutrons, about which electrons orbit
7.
(chem) a fundamental group of atoms in a molecule serving as the base structure for related compounds and remaining unchanged during most chemical reactions: the benzene nucleus
8.
(botany)

9.
(phonetics) the most sonorous part of a syllable, usually consisting of a vowel or frictionless continuant
10.
(logic) the largest individual that is a mereological part of every member of a given class
n.

1704, “kernel of a nut,” 1708, “head of a comet,” from Latin nucleus “kernel,” from nucula “little nut,” diminutive of nux (genitive nucis) “nut,” from PIE *kneu- “nut” (cf. Middle Irish cnu, Welsh cneuen, Middle Breton knoen “nut,” Old Norse hnot, Old English hnutu “nut”). General sense of “central part or thing, about which others cluster” is from 1762. Use in reference to cells first recorded 1831. Modern atomic meaning is 1912, first by Ernest Rutherford, though theoretical use for “central point of an atom” is from 1844, in Faraday.

nucleus nu·cle·us (nōō’klē-əs, nyōō’-)
n. pl. nu·cle·us·es or nu·cle·i (-klē-ī’)

nucleus
(n’klē-əs)
Plural nuclei (n’klē-ī’)

plur. nuclei (nooh-klee-eye)

The small, dense center of the atom. The nucleus is composed of protons and neutrons and has a positive electrical charge.

Note: Nuclear physics deals with the composition and structure of the nucleus.

plur. nuclei

In biology, the central region of the cell, in which DNA is stored. The nucleus usually appears as a dark spot in the interior of the cell. Primitive cells (such as bacteria and blue-green algae) have no nuclei.