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nucleus Syllabification: nu·cle·us
Pronunciation: /ˈn(y)o͞oklēəs/

Definition of nucleus in English:

noun (plural nuclei-klēˌī)

1The central and most important part of an object, movement, or group, forming the basis for its activity and growth: the nucleus of a film-producing industry
More example sentences
  • If this proposal is accepted, the government together with the St Lucian organizations could have an extension of the mission in Brooklyn and an important nucleus for the St Lucian community.
  • The nucleus of the global movement against injustice will not - cannot, should not - be found in the centers of global privilege.
  • And at the moment there is not even the nucleus of a movement to achieve that.
core, center, central part, heart, nub, hub, middle, eye, focus, focal point, pivot, crux
1.1 Physics The positively charged central core of an atom, consisting of protons and neutrons and containing nearly all its mass.
Example sentences
  • The harmless radio waves excite protons that form the nuclei of hydrogen atoms in the body.
  • Electrons produce a small magnetic field as they spin and orbit the nucleus of an atom.
  • Electrons are found in clouds that surround the nucleus of an atom.
1.2 Biology A dense organelle present in most eukaryotic cells, typically a single rounded structure bounded by a double membrane, containing the genetic material.
Example sentences
  • Dense granules surrounding the nucleus are probably cytoplasmic organelles.
  • The cells had round or oval vesicular nuclei with multiple prominent nucleoli.
  • The cells were characterized by large vesicular nuclei with single large nucleoli.
1.3 Astronomy The solid part of the head of a comet.
Example sentences
  • Sometimes, the stresses are strong enough to break off chunks of the comet's nucleus.
  • The spacecraft will orbit the comet's nucleus.
  • The spacecraft's point of view now captures the shadowed side of the comet's nucleus.
1.4 Anatomy A discrete mass of gray matter in the central nervous system.
Example sentences
  • Axons travel from the neurons located in nuclei within the brain stem via the cranial nerves without synaptic interruption to the motor end plates on the striated muscle.
  • These effects are produced by fibres projecting from the hypothalamus to parasympathetic nuclei in the brain stem, and to sympathetic centres in the spinal cord.
  • The trochlear nerve nuclei are located ventral to the mesencephalic aqueduct.


Early 18th century: from Latin, literally 'kernel, inner part', diminutive of nux, nuc- 'nut'.

  • The nucleus of something is literally its ‘little nut’. In Latin nucleus meant ‘kernel, inner part’, and was a diminutive of nux ‘nut’. Nucleus originally referred to the bright core at the centre of a comet's head, and then to the central part of the earth. Today its main technical meaning is ‘the positively charged central core of an atom’. This was identified by Sir Ernest Rutherford ( 1871–1937), regarded as the founder of nuclear physics, in 1911.

Words that rhyme with nucleus

coleus • Julius • Equuleus

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