A functional unit consisting of a promoter, an operator and a number of structural genes, found mainly in prokaryotes. The structural genes commonly code for several functionally related enzymes, and although they are transcribed as one (polycistronic) mRNA each is independently translated. In the typical operon, the operator region acts as a controlling element in switching on or off the synthesis of mRNA.
PAC, 1992, 64, 143 (Glossary for chemists of terms used in biotechnology (IUPAC Recommendations 1992)) on page 162