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nu·cle·us (nklē-əs, ny-)
n. pl. nu·cle·i (-klē-ī) or nu·cle·us·es
1. A central or essential part around which other parts are gathered or grouped; a core: the nucleus of a city.
2. Something regarded as a basis for future development and growth; a kernel: a few paintings that formed the nucleus of a great art collection.
3. Biology A membrane-bound organelle within a eukaryotic cell that contains most of the cell's genetic material. DNA transcription takes place in the nucleus.
4. Anatomy A group of specialized nerve cells or a localized mass of gray matter in the brain or spinal cord.
5. Physics The positively charged central region of an atom, composed of protons and neutrons, about which negatively charged electrons orbit. Extremely small and dense, the nucleus contains almost all of the mass of an atom.
6. Chemistry A group of atoms bound in a structure, such as a benzene ring, that is resistant to alteration in chemical reactions.
7. Astronomy
a. The central, often brightest part of the head of a comet.
b. The solid part of a comet, composed of ice and smaller amounts of dust and rock.
c. The central, often brightest part of a galaxy.
8. Meteorology A particle on which water vapor molecules accumulate in free air to form a droplet or ice crystal.
9. Linguistics The part of a syllable having the greatest sonority. In the word middlemost (mĭdl-mōst) the nuclei of the three syllables are (ĭ), (l), and (ō); in the Czech word krk ("neck"), the nucleus is (r).

[Latin nuculeus, nucleus, kernel, from nucula, little nut, diminutive of nux, nuc-, nut.]

The American Heritage® Dictionary of the English Language, Fifth Edition copyright ©2022 by HarperCollins Publishers. All rights reserved.

Indo-European & Semitic Roots Appendices

    Thousands of entries in the dictionary include etymologies that trace their origins back to reconstructed proto-languages. You can obtain more information about these forms in our online appendices:

    Indo-European Roots

    Semitic Roots

    The Indo-European appendix covers nearly half of the Indo-European roots that have left their mark on English words. A more complete treatment of Indo-European roots and the English words derived from them is available in our Dictionary of Indo-European Roots.