WordNet (r) 3.0 (2006):

ribosome
    n 1: an organelle in the cytoplasm of a living cell; they attach
         to mRNA and move down it one codon at a time and then stop
         until tRNA brings the required amino acid; when it 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"