nucleus

noun

nu·​cle·​us ˈnü-klē-əs 

ˈnyü-

plural nuclei ˈnü-klē-ˌī 

ˈnyü-

also nucleuses

1

a

: the small bright body in the head of a comet

b

: the small brighter and denser portion of a galaxy (see galaxy sense 1b)

2

: a central point, group, or mass about which gathering, concentration, or accretion takes place: such as

a

: a cellular organelle of eukaryotes that is essential to cell functions (such as reproduction and protein synthesis), is composed of nucleoplasm and a nucleoprotein-rich network from which chromosomes and nucleoli arise, and is enclosed in a definite membrane see cell illustration

b

: a mass of gray matter or group of cell bodies of neurons in the central nervous system

c

: a characteristic and stable complex of atoms or groups in a molecule (see molecule sense 1)

especially : ring

the naphthalene nucleus

d

: the positively charged central portion of an atom that comprises nearly all of the atomic mass and that consists of protons and usually neutrons

3

: the peak of sonority in the utterance of a syllable

4

: a basic or essential part : core

players who are the nucleus of the team

Synonyms

• axis
• base
• capital
• center
• central
• core
• cynosure
• epicenter
• eye
• focus
• ground zero
• heart
• hub
• locus
• mecca
• navel
• nerve center
• nexus
• omphalos
• seat

Examples of nucleus in a Sentence

a college campus that was a nucleus of opposition to the war

the nucleus of the movement's methodology has always been passive resistance

Recent Examples on the Web

The lead atom's nucleus vibrates and ejects three of its protons, in effect turning into a gold atom. —George Petras, USA Today, 17 May 2025

This pulse can trigger a process known as electromagnetic dissociation, in which a photon interacts with a nucleus, inducing internal oscillations that eject neutrons and photons. —Victoria Corless, Space.com, 13 May 2025

That is, a fusion reaction that fuses a hydrogen nucleus and a boron-11 atom instead of two atoms of the hydrogen isotopes deuterium and tritium. —David Szondy may 10, New Atlas, 10 May 2025

In rare cases, three protons are knocked out of a lead nucleus, leaving behind gold in its place. —Melissa Fleur Afshar, MSNBC Newsweek, 9 May 2025

Word History

Etymology

New Latin, from Latin, kernel, from nuc-, nux nut — more at nut

First Known Use

1668, in the meaning defined at sense 1a

Phrases Containing nucleus

• caudate nucleus
• endosperm nucleus
• generative nucleus
• polar nucleus
• sperm nucleus
• suprachiasmatic nucleus
• tube nucleus

Rhymes for nucleus

• micronucleus

Kids Definition

nucleus

noun

nu·​cle·​us ˈn(y)ü-klē-əs 

plural nuclei -klē-ˌī  also nucleuses

: a central point, group, or mass of something: as

a

: the small, brighter, and denser part of a galaxy or of the head of a comet

b

: a cell part that is characteristic of all living things with the exception of viruses, bacteria, and blue-green algae, that is necessary for heredity and for making proteins, that contains the chromosomes, and that is enclosed in a nuclear membrane

c

: a mass of gray matter or group of neurons in the central nervous system

d

: the central part of an atom that includes nearly all of the atomic mass and consists of protons and usually neutrons

Etymology

from modern Latin nucleus "the central part of something," from Latin nucleus "kernel," derived from nux "nut"

Medical Definition

nucleus

noun

nu·​cle·​us ˈn(y)ü-klē-əs 

plural nuclei -klē-ˌī  also nucleuses

1

: a cellular organelle of eukaryotes that is essential to cell functions (as reproduction and protein synthesis), is composed of nuclear sap and a nucleoprotein-rich network from which chromosomes and nucleoli arise, and is enclosed in a definite membrane

2

: a mass of gray matter or group of nerve cells in the central nervous system

3

: a characteristic and stable complex of atoms or groups in a molecule

especially : ring sense 2

the naphthalene nucleus

4

: the positively charged central portion of an atom that comprises nearly all of the atomic mass and that consists of protons and neutrons except in hydrogen which consists of one proton only