1) Within most types of eukaryotic cell nucleus, a distinct region, not delimited by a membrane, in which some species of rRNA (RNA, ribosomal) are synthesized and assembled into ribonucleoprotein subunits of ribosomes. In the nucleolus rRNA is transcribed from a nucleolar organizer, i.e., a group of tandemly repeated chromosomal genes which encode rRNA and which are transcribed by RNA polymerase I.

2) A spherical body of the nucleus of most eukaryotes that becomes enlarged during protein synthesis, is associated with a nucleolus organizer, and contains the DNA templates for ribosomal RNA -- called also plasmosome.

3) The nucleolus is a region found within the cell nucleus that is concerned with producing and assembling the cell's ribosomes. Following assembly, ribosomes are transported to the cell cytoplasm where they serve as the sites for protein synthesis.