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 large, membrane-bound, usually spherical protoplasmic structure within a living cell, containing the cell's hereditary material and controlling its metabolism, growth, and reproduction.

4. Botany. a. The central kernel of a nut or seed. b. The center of a starch granule.

5. Anatomy. A group of specialized nerve cells or a localized mass of gray matter in the brain or spinal cord.

6. Physics. The positively charged central region of an atom, composed of protons and neutrons and containing almost all of the mass of the atom.

7. Chemistry. A group of atoms bound in a structure, such as a benzene ring, that is resistant to alteration in chemical reactions.

8. Astronomy. a. The central portion of the head of a comet. b. The central or brightest part of a nebula or galaxy.

9. Meteorology. A particle on which water vapor molecules accumulate in free air to form a droplet or ice crystal.