Master Chief Petty Officer
A Master Chief Petty Officer is a military rank the equivalent of which is used by the service organizations of many civilizations.
Use in Starfleet
Master Chief Petty Officer is the most senior regular non-commissioned rank currently used by Starfleet. The Marine rank equivalent to Master Chief Petty Officer is Sergeant Major.
Master Chief Petty Officers generally only serve as department heads; or, more commonly, as the Chief of Boat. They have many years of service and a wealth of experience in their selected field. There is usually only one Master Chief Petty Officer assigned to a starship at any one time, although it is possible to have more then one Master Chief Petty Officer if the other one serves in a department head position. Master Chief Petty Officers are at the top of the Enlisted/NCO ladder and can therefore not be promoted further; although they can apply for a warrant or commission.