| M. | Master (pilot, etc.); mate; medical; mine-sweeper. |
| M.A. | Military Assistant. |
| M.A.A. | Master at Arms. |
| M.A.A.F. | Mediterranean Allied Air Forces. |
| M.A.C. | Merchant aircraft carrier; motor ambulance convoy. |
| M.A.E.E. | Marine Aircraft Experimental Establishment. |
| Maj. | Major. |
| Maj.-Gen. | Major-General. |
| M. & D. | Medicine and duty. |
| M. & T. | Movements and Transportation. |
| M. & V. | Meat and vegetables. |
| M.A.P. | Medical Aid Post; Ministry of Aircraft Production. |
| M.A.R.U. | Mobile Aircraft Reporting Unit. |
| M.A.T.S. | Mediterranean Air Transport Service. |
| M.B. | Motor-boat. |
| M.B.O. | Medical Battle Organization. |
| Me. | Marine Craft Officer (R.A.F.). |
| M.C. | Military Cross; Movement Control. |
| M c. | Motor cycle. |
| M.C.O. | Movement Control Officer. |
| M.C.S. | Military College of Science. |
| M.D. | Medical Department (R.N.); Mines Department (R.N.). |
| M.D.S. | Main Dressing Station. |
| M.D.U. | Mobile Dental Unit. |
| Me. | Messerschmitt. |
| M.E. | Muzzle energy. |
| Mech. | Mechanic. |
| Med. | Medical; medium. |
| MEDME. | Mediterranean and Middle East. |
| M.E.F. | Middle East Forces. |
| M.E.L.F. | Middle East Land Forces. |
| M.E.S. | Military Engineer Services. |
| Met. | Meteorological. |
| M.E.W. | Ministry of Economic Warfare. |
| M.F.H. | Mobile Field Hospital. |
| M.F.O. | Military Forwarding Officer. |
| M.G. | Machine-gun; Major-General. |
| M.G.A. | Major-General, Administration. |
| M.G.B. | Motor gunboat. |
| M.G.G.S. | Major-General, General staff. |
| M.G.O. | Master-General of Ordnance. |
| M.I. | Medical inspection; military intelligence. |
| Mid. | Midshipman. |
| M.i.h. | Miles in the hour. |
| Mil | Military; Militia. |
| Mk. | Mark. |
| M.L. | Minelayer; motor launch. |
| M.L.O. | Military Landing Officer. |
| M.L.U. | Mobile Landing Unit. |
| M.M. | Military Medal; motor mechanic. |
| M.M.L. | Manual of Military Law. |
| M.N. | Merchant Navy. |
| M.O. | Medical Officer; Meteorological Officer. |
| Mob. | Mobile. |
| Mob.B.U. | Mobile Bath Unit. |
| M. of F. | Ministry of Food. |
| M.O.I. | Ministry of Information. |
| Mon. | Monitor (R.N.). |
| M.O.P. | Ministry of Production. |
| M.O.R.U. | Mobile Operations Room Unit. |
| M.O.S. | Ministry of Supply. |
| M.P. | Military Police. |
| M.P.I. | Mean point of impact. |
| M.P.S.C. | Military Provost Staff Corps. |
| M.R. | Map reference. |
| M.R.A.F. | Marshal of the Royal Air Force. |
| M.R.S. | Medical Receiving Station. |
| M.S. | Medical Services; Military Secretary; minesweeper. |
| M.S.A. | Military Service Act. |
| M.S.F.U. | Merchant Ships Fighter Unit. |
| M.S.M. | Meritorious Service Medal. |
| M.T. | Mechanical transport; military training. |
| M.T. and S. | Mechanical transport and supply. |
| M.T.B. | Motor torpedo boat. |
| M.T.M. | Mechanical transport mechanic. |
| M.T.O. | Mechanical Transport Officer. |
| M.U. | Maintenance Unit; mobile unit. |
| M.V. | Motor vehicle; motor vessel; muzzle velocity. |
| M.W.T. | Ministry of War Transport. |
| M.Y. | Motor yacht. |