- Plural: nuclei
- 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 nucleus) 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, nucleus can refer to
- In computing, the nucleus can refer to an operating system kernel and is a brand of operating system itself (Nucleus RTOS).
'Nucleus' is New Latin, diminutive of the Latin nux (nut).