Source : Webster's Revised Unabridged Dictionary (1913)

Nucleolus \Nu*cle"o*lus\, n.; pl. {Nucleoli}. [L., a little nut,
   dim. of nucleus.]
   1. A little nucleus.

   2. (Biol.) A small rounded body contained in the nucleus of a
      cell or a protozoan.

   Note: It was termed by Agassiz the entoblast. In the
         protozoa, where it may be situated on one side of the
         nucleus, it is sometimes called the endoplastule, and
         is supposed to be concerned in the male part of the
         reproductive process. See {Nucleus}.

Source : WordNet®

nucleolus
     n : a small round body of protein in a cell nucleus; nucleoli
         contain RNA and are involved in protein synthesis [syn: {nucleole}]
     [also: {nucleoli} (pl)]