nucleus

noun nu·cle·us \ˈnü-klē-əs, ˈnyü-\

biology : the central part of most cells that contains genetic material and is enclosed in a membrane

physics : the central part of an atom that is made up of protons and neutrons

: a central or most important part of something

plural nu·clei \-klē-ˌī\ also nu·cle·us·es

Full Definition of NUCLEUS

a : the small bright body in the head of a comet

b : the small brighter and denser portion of a galaxy

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

a : a cellular organelle of eukaryotes that is essential to cell functions (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; 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

: the peak of sonority in the utterance of a syllable

: a basic or essential part : core <players who are the nucleus of the team>

See nucleus defined for English-language learners

See nucleus defined for kids

Origin of NUCLEUS

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

First Known Use: 1704

Related to NUCLEUS

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

Other Cell Biology Terms

cytology, gamete, vacuole, zygote

NUCLEI Defined for Kids

nucleus

noun nu·cle·us \ˈnü-klē-əs, ˈnyü-\

plural nu·clei \-klē-ˌī\

Definition of NUCLEUS for Kids

: a usually round part of most cells that is enclosed in a double membrane, controls the activities of the cell, and contains the chromosomes

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

: a central point, group, or mass <Those players are the nucleus of the team.>

Medical Dictionary

nucleus

noun nu·cle·us \ˈn(y)ü-klē-əs\

plural nu·clei \-klē-ˌī\ also nu·cle·us·es

Medical Definition of NUCLEUS

: 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

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

: a characteristic and stable complex of atoms or groups in a molecule; especially : ring 2 <the naphthalene nucleus>

: 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