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]

Moby Thesaurus II by Grady Ward, 1.0:

112 Moby Thesaurus words for "nucleus":
   Anlage, Kern, NMR, alpha particle, antibaryon, antilepton,
   antimeson, atomic nucleus, atomic particle, axiom, axis, baryon,
   basichromatin, beta particle, bud, center, center of action,
   center of gravity, centroid, centrum, chromatin, chromatin strands,
   core, dead center, diameter, diaphragm, egg, electron,
   elementary particle, elixir, embryo, epicenter, equator, essence,
   essential, flower, focus, fundamental, germ, germen, gist,
   gravamen, heart, heterochromatin, hub, hypostasis, inner essence,
   interior, karyosome, karyotin, kernel, lepton, loins, macronucleus,
   marrow, mean, meat, median, medulla, meganucleus, meson,
   metacenter, micronucleus, middle, midmost, midriff, midst, nave,
   navel, neutron, nub, nuclear force, nuclear magnetic resonance,
   nuclear particle, nuclear resonance, nucleolus, nucleon,
   nucleosynthesis, nuts and bolts, omphalos, ovum, photon, pith,
   pivot, plasmosome, postulate, principle, proton, quid, quiddity,
   quintessence, rudiment, sap, seed, soul, spark, spermatozoon,
   spirit, storm center, strangeness, strong interaction, stuff,
   substance, the nitty-gritty, thick, thick of things, triton,
   umbilicus, valence electron, waist, waistline, zone