The Collaborative International Dictionary of English v.0.48:

Nucleus \Nu"cle*us\, n.; pl. E. Nucleuses, L. Nuclei. [L., a
   kernel, dim. fr. nux, nucis, nut. Cf. Newel post.]
   [1913 Webster]
   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.
      [1913 Webster]

            It must contain within itself a nucleus of truth.
                                                  --I. Taylor.
      [1913 Webster]

   2. (Astron.) The body or the head of a comet.
      [1913 Webster]

   3. (Bot.)
      (a) An incipient ovule of soft cellular tissue.
      (b) A whole seed, as contained within the seed coats.
          [1913 Webster]

   4. (Biol.) A body, usually spheroidal, in a eukaryotic cell,
      distinguished from the surrounding protoplasm by a
      difference in refrangibility and in behavior towards
      chemical reagents, which contains the chromosomal genetic
      material, including the chromosomal DNA. 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.
      [1913 Webster +PJC]

   Note: The nucleus is sometimes termed the endoplast or
         endoblast, and in the protozoa is supposed to be
         concerned in the female part of the reproductive
         process. See Karyokinesis.
         [1913 Webster]

   5. (Zool.)
      (a) The tip, or earliest part, of a univalve or bivalve
          shell.
      (b) The central part around which additional growths are
          added, as of an operculum.
      (c) A visceral mass, containing the stomach and other
          organs, in Tunicata and some mollusks.
          [1913 Webster]

WordNet (r) 3.0 (2006):

nucleus
    n 1: a part of the cell containing DNA and RNA and responsible
         for growth and reproduction [syn: nucleus, 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, nucleus, 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
    6: the central structure of the lens that is surrounded by the
       cortex [syn: lens nucleus, nucleus]