Source : Webster's Revised Unabridged Dictionary (1913)

Nucleus \Nu"cle*us\, n.; pl. E. {Nucleuses}, L. {Nuclei}. [L., a
   kernel, dim. fr. nux, nucis, nut. Cf. {Newel} post.]
   1. 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.

            It must contain within itself a nucleus of truth.
                                                  --I. Taylor.

   2. (Astron.) The body or the head of a comet.

   3. (Bot.)
      (a) An incipient ovule of soft cellular tissue.
      (b) A whole seed, as contained within the seed coats.

   4. (Biol.) A body, usually spheroidal, in a cell or a
      protozoan, distinguished from the surrounding protoplasm
      by a difference in refrangibility and in behavior towards
      chemical reagents. It is more or less protoplasmic, and
      consists of a clear fluid (achromatin) through which
      extends a network of fibers (chromatin) in which may be
      suspended a second rounded body, the nucleolus (see
      {Nucleoplasm}). See {Cell division}, under {Division}.

Source : WordNet®

nucleus
     n 1: a part of the cell containing DNA and RNA and responsible
          for growth and reproduction [syn: {cell nucleus}, {karyon}]
     2: the positively charged dense center of an atom
     3: a small group of indispensable persons or things; "five
        periodicals make up the core of their publishing program"
        [syn: {core}, {core group}]
     4: (astronomy) the center of the head of a comet; consists of
        small solid particles of ice and frozen gas that vaporizes
        on approaching the sun to form the coma and tail
     5: any histologically identifiable mass of neural cell bodies
        in the brain or spinal cord
     [also: {nuclei} (pl)]