From 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.]
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   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
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            It must contain within itself a nucleus of truth.
                                                  --I. Taylor.
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   2. (Astron.) The body or the head of a comet.
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   3. (Bot.)
      (a) An incipient ovule of soft cellular tissue.
      (b) A whole seed, as contained within the seed coats.
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   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.
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   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.
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   5. (Zool.)
      (a) The tip, or earliest part, of a univalve or bivalve
      (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]
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