nucleus

American  

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

noun

nuclei, plural nucleuses plural

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

   A few faithful friends formed the nucleus of the club.

   Synonyms:

   heart, kernel, center

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.

   1. 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.

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

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nucleus British  

/ ˈnjuːklɪəs /

noun

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

   1. the central point of a starch granule

   2. a rare name for nucellus

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

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nucleus Scientific  

/ no̅o̅′klē-əs /

nuclei plural

1. The positively charged central region of an atom, composed of one or more protons and (for all atoms except hydrogen) one or more neutrons, containing most of the mass of the atom. The strong force binds the protons and neutrons, also known as nucleons, to each other, overcoming the mutual repulsion of the positively charged protons. In nuclei with many nucleons, however, the forces of repulsion may overcome the strong force, and the nucleus breaks apart in the process of radioactive decay. The protons and neutrons are arranged in the nucleus in energy levels known as shells analogous to those of the electrons orbiting the nucleus. The number of protons in the nucleus determines the atom's atomic number and its position in the Periodic Table.

2. See more at atom

3. An organelle in the cytoplasm of eukaryotic cells (all cells except prokaryotes) that contains nearly all the cell's DNA and controls its metabolism, growth, and reproduction. The nucleus is surrounded by a pair of membranes called the nuclear envelope, which can be continuous in places with the membranes of the endoplasmic reticulum. The membranes of the nuclear envelope have interconnected pores that allow the exchange of substances with the cell's cytoplasm. The nuclear DNA is wrapped around proteins (called histones) in strands of chromatin, which exists in a matrix known as nucleoplasm (analogous to the cytoplasm outside the nucleus). Just prior to cell division, the chromatin condenses into individual chromosomes, which contain the cell's hereditary information. The nucleus also contains at least one spherical nucleolus, which mainly contains RNA and proteins and directs the construction of the cell's ribosomes.

4. See more at cell

5. 1. The solid central part of a comet, typically several kilometers in diameter and composed of ice, frozen gases, and embedded chunks of rock and dust. It is the permanent part of a comet from which the coma and tail are generated as the comet approaches the Sun.

   2. See galactic nucleus

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nucleus ¹ Cultural  

1. 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.

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nucleus ² Cultural  

1. 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.

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Discover More

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

Other Word Forms

• subnucleus
  noun

Etymology

Origin of nucleus

First recorded in 1695–1705; from Latin: “kernel,” shortened variant of nuculeus, from nucu(la) “little nut” ( nuc-, stem of nux “nut” + -ula -ule ) + -leus, noun suffix

Explanation

A nucleus is often called the brain, or control center, of a eukaryotic cell (that is, a cell with a nucleus). The noun nucleus has several senses, though most of them will appear in science. If a biology teacher says nucleus, she is probably talking about a cell (or a group of cells), but a physics teacher would be talking about the center of an atom, or possibly even the center of a comet’s head. Step out of science class, and nucleus likely refers to a core group — like the math fanatics that math club could not function without.

Example Sentences

Since taking the leap to living in the L.A. area in 2014, she has exhibited her work at Gallery Nucleus, Corey Helford Gallery and La Luz de Jesus Gallery, to name a few.

From Los Angeles Times • Mar. 25, 2026

Sadeghi said that Nucleus has served thousands of families, extending “well beyond the walls of Silicon Valley.”

From The Wall Street Journal • Nov. 9, 2025

Nucleus, which has raised $32 million, is backed by venture-capital firms founded by Thiel and Ohanian and charges $9,999 to provide polygenic screening on up to 20 embryos.

From The Wall Street Journal • Nov. 9, 2025

Phunware did not answer specific questions about the partnership with Nucleus, saying the company’s client engagements were confidential.

From Seattle Times • May 5, 2024

One fragment, known as Nucleus G, struck with the force of about six million megatons–seventy-five times more than all the nuclear weaponry in existence.

From "A Short History of Nearly Everything" by Bill Bryson