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nucleus

n.
US //ˈnukɫiəs// UK //njˈuːklɪəs// nu·cle·us
  1. 1 biology (n.)
    B2 Upper Intermediate Technical Science

    the central part of a living cell that contains its genetic information and controls its growth.

    a membrane-bound organelle within eukaryotic cells containing the genetic material and governing cellular activity.

    Example

    The biology teacher showed us how the nucleus holds the DNA of the cell.

    Example

    Under the microscope, the nucleus appeared as a dense, dark-stained sphere, clearly separated from the surrounding cytoplasm by its own membrane.

    Usage

    The plural form is 'nuclei'.
  2. 2 physics/chemistry (n.)
    B2 Upper Intermediate Technical Science

    the small, heavy center of an atom that contains protons and neutrons.

    the positively charged central core of an atom, consisting of protons and neutrons and containing nearly all its mass.

    Example

    Energy is released when the nucleus of a heavy atom is split.

    Example

    The stability of an isotope depends largely on the ratio of neutrons to protons within its nucleus.

    Usage

    The plural form is 'nuclei'.
    • atomic core
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  1. 3 the core of a group (n.)
    C1 Advanced Formal

    the small, central group of people or things that a larger group grows around.

    the central or essential part around which other parts are gathered or a group is formed.

    Example

    A small nucleus of dedicated volunteers started the charity ten years ago.

    Example

    The original research team formed the nucleus of the new department, providing the expertise necessary to attract further funding.

    Teacher's tip

    In this figurative sense, it describes the 'seed' or 'starting point' of a larger organization.
  2. 4 linguistics (n.)
    C2 Proficiency Technical

    the central part of a syllable, which is usually a vowel sound.

    the central part of a syllable, typically consisting of a vowel or a syllabic consonant.

    Example

    In the word 'cat', the vowel 'a' is the nucleus of the syllable.

    Example

    Phonologists distinguish between the onset of a syllable and its rhyme, the latter of which is composed of the nucleus and an optional coda.
Origin

Learned borrowing from Latin nucleus (“kernel, core”). The earliest uses refer to the head of a comet and the kernel of a seed, both recorded in Lexicon Technicum in 1704. The sense in atomic physics was coined by British scientist Michael Faraday in 1844 in a theoretical meaning.

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