Definition of: nucleus

(n′klē·əs, ny′-) noun plural ·cle·i (-klē·ī)
1. A center of development; central mass; kernel.
2. Biol. A complex, spheroidal body surrounded by a thin membrane and embedded in the protoplasm of most plant and animal cells. It contains the chromatin which is essential in the processes of heredity, and is the directive center of all the vital activities of the cell, as assimilation, metabolism, growth, and reproduction.
3. Physiol. A group of nerve cells within the nervous system from which the nerve fibers originate.
4. Zool. The apex, or earliest formed part of a shell; also, the central part, as of an operculum, around which additional parts are formed.
5. Astron. The starlike point seen in the head of a comet, and at the center of a nebula.
6. Physics The central core of an atom, believed to contain its effective mass and to have a positive charge balanced by the negative charge of the surrounding electrons. Its principal components are the proton and neutron. [<L, a kernel, dim. of nux, nucis a nut]