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: Webster's Revised Unabridged Dictionary (1913)