A kernel; hence, a central mass or point about which matter is gathered, or to which accretion is made; the central or material portion; -- used both literally and figuratively.
- A central or essential part around which other parts are gathered or grouped; a core: the nucleus of a city.
- Something regarded as a basis for future development and growth; a kernel: a few paintings that formed the nucleus of a great art collection.
- Biology A large, membrane-bound, usually spherical protoplasmic structure within a living cell, containing the cell's hereditary material and controlling its metabolism, growth, and reproduction.
- Botany The central kernel of a nut or seed.
- Botany The center of a starch granule.
- Anatomy A group of specialized nerve cells or a localized mass of gray matter in the brain or spinal cord.
- Physics The positively charged central region of an atom, composed of protons and neutrons and containing almost all of the mass of the atom.
- Chemistry A group of atoms bound in a structure, such as a benzene ring, that is resistant to alteration in chemical reactions.
- Astronomy The central portion of the head of a comet.
- Astronomy The central or brightest part of a nebula or galaxy.
- Meteorology A particle on which water vapor molecules accumulate in free air to form a droplet or ice crystal.
- Linguistics The part of a syllable having the greatest sonority. In the word middlemost (mĭdˈl-mōstˌ) the nuclei of the three syllables are (ĭ), (l), and (ō); in the Czech word krk ("neck”), the nucleus is (r).