The Collaborative International Dictionary of English v.0.48:

Nucleolus \Nu*cle"o*lus\, n.; pl. Nucleoli. [L., a little nut,
   dim. of nucleus.]
   1. A little nucleus.
      [1913 Webster]

   2. (Biol.) A small rounded body contained in the nucleus of a
      cell or a protozoan.
      [1913 Webster]

   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.
         [1913 Webster]

WordNet (r) 3.0 (2006):

nucleolus
    n 1: a small round body of protein in a cell nucleus; such
         organelles contain RNA and are involved in protein
         synthesis [syn: nucleolus, nucleole]