- In chemistry and physics, the nucleus (atomic nucleus) is the collection of protons and neutrons in the center of an atom that carries the bulk of the atom's mass and positive charge.
- In cell biology, the nucleus (cell nuclus) is the membrane-bound subcellular organelle found in eukaryotes, visible via microscopy, which contains, primarily, the cell's chromosomes.
- In neuroanatomy, a nucleus is a central nervous system structure that is composed mainly of gray matter and mediates electrical signaling within a particular subsystem.
- In comets, a nucleus (comet nucleus) is the solid core of a comet.
- In astronomy, a nucleus (galaxy nucleus) is the central region of a galaxy.
- In meteorology, a nucleus (ice nucleus) is the center of an ice crystal.
- In meteorology, a nucleus (cloud condensation nuclei) is the basis for the development of a cloud droplet.
- In linguistics, the nucleus is the center part of a syllable, most often a vowel.
Etymology"Nucleus" comes from the Latin for center or core.
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