| MT | magnetic particle test |
| MCES | main condenser evacuation system |
| MCB | main control board |
| MFW | main feedwater |
| MFWP | main feedwater pump |
| MSIV | main steam isolation valve |
| MSLB | main steam line break |
| MSSS | main steam supply system |
| makeup (noun, adj), make up (verb) |
| MAS | Management Action System (PIMS) |
| mgr | manager |
| manhour |
| manpower |
| man-rem |
| mfg | manufacturing |
| manways |
| margin of safety |
| markup (noun), marked-up (adj) |
| max | maximum |
| MPC | maximum permissible concentration |
| Mx | maxwell |
| MLLW | mean lower low water |
| MSL | mean sea level |
| M&TE | measurement and test equipment |
| mech | mechanical |
| MM | Mechanical Maintenance |
| M | mega- (106) (prefix) |
| MCi | megacurie |
| Mc | megacycle |
| MHz | megahertz (megacycles per second) |
| MΩ | megohm |
| MV | megavolt |
| MVA | megavolt-ampere |
| MW | megawatt |
| MWd/kgU or MWD/KGU | megawatt days per kilogram of uranium |
| MWd/mtU or MWD/MTU | megawatt days per metric ton of uranium |
| MWe | megawatt, electrical |
| MWh | megawatt-hour |
| MWt | megawatt, thermal |
| MOA | memorandum of agreement (NRC) |
| metalclad |
| m | meter(s) |
| MKS | meter-kilogram-second (system of units) |
| m/s | meters per second |
| mho |
| : | micro- (10-6) (prefix) |
| :A | microampere |
| :A | microangstrom |
| :C | microcoulomb |
| :Ci | microcurie |
| :F | microfarad |
| :g | microgram |
| :H | microhenry |
| :L | microliter |
| :m | micrometer |
| :mho | micromho |
| :: | micromicron (10-12 m) |
| :M or :mol | micromole |
| : | micron (10-6 m) |
| :s | microsecond |
| :V | microvolt |
| :W | microwatt |
| MOL | middle of life |
| midloop |
| midpower |
| midrange |
| midscale |
| mph | miles per hour |
| m | milli- (10-3) (prefix) |
| mA | milliampere |
| mCi | millicurie |
| mF | millifarad |
| mg | milligram(s) |
| mH | millihenry |
| mL | milliliter(s) |
| mm | millimeter(s) |
| mmho | millimho |
| MCM | million circular mills |
| MeV | million electron volts |
| MV | million volts |
| mRem | millirem |
| mR | milliroentgen |
| ms | millisecond |
| mV | millivolt |
| mW | milliwatt |
| min | minimum |
| MCHFR | minimum critical heat flux ratio |
| MCPR | minimum critical power ratio |
| min | minute(s) |
| misc | miscellaneous |
| Mode l (Power Operation) |
| Mode 2 (Startup) |
| Mode 3 (Hot Standby) |
| Mode 4 (Hot Shutdown) |
| Mode 5 (Cold Shutdown) |
| Mode 6 (Refueling) |
| MELB | moderate energy line break |
| MTC | moderator temperature coefficient |
| MAAP | Modular Accident Analysis Program |
| MSR | moisture separator-reheater |
| mol wt | molecular weight |
| MRS | monitored retrievable storage |
| MFP | Mothers for Peace |
| MCC | motor control center |
| MG | motor generator |
| MOV | motor-operated valve |
| multichannel |
| multiconductor |
| multi-degree-of-freedom |
| multiplex |
| MPC | multi-purpose canister |
| multivariable |
