Source : WordNet®

ribosome
     n : an organelle in the cytoplasm of a living cell; ribosomes
         attach to mRNA and move down it one codon at a time and
         stop until tRNA brings the required amino acid; when a
         ribosome reaches a stop codon it falls apart and releases
         the completed protein molecule for use by the cell; "the
         ribosome is the site of protein synthesis"