1) The most abundant form of RNA. Together with proteins, it forms the ribosomes, playing a structural role and also a role in ribosomal binding of mRNA and tRNAs. Individual chains are conventionally designated by their sedimentation coefficients. In eukaryotes, four large chains exist, synthesised in the nucleolus and constituting about 50% of the ribosome. (Dorland, 28th ed)

2) A class of RNA found in the ribosomes of cells.