| M | Mega - One Thousand (in Roman Numerals) | - | Astronomy |
| M | Metre - Milli | - | Astronomy |
| M&S | Modelling & Simulation - Marks & Spencer | - | Astronomy |
| MA | Master Of Arts -Madagascar (FIPS 10-4 Country Code) - Massachusetts (postal Symbol) - Megaampere - Morocco (ISO 3166 Digram) | - | Astronomy |
| MA | Megaannum | - | Astronomy |
| MA | Milliampere | - | Astronomy |
| MAA | Mathematical Association Of America | - | Astronomy |
| MAAMAW | Modelling Autonomous Agents In A Multi-Agent World | - | Astronomy |
| MAC | Macao (ISO 3166 Trigram) -U.S. Military Airlift Command (1966-1992) | - | Astronomy |
| MACA | Military Aid To The Civil Authorities | - | Astronomy |
| MACE | Multi-Agent Computing Environment | - | Astronomy |
| MACHO | MAssive Compact Halo Object | - | Astronomy |
| MAD | Magnetic Anomaly Detector -Moroccan Dirham (ISO 4217 Currency Code) -Mutual Assured Destruction | - | Astronomy |
| MADD | Mothers Against Drunk Driving - MyoAdenylate Deaminase Deficiency -Multiply-Add | - | Astronomy |
| MAF | Maintenance Action Form | - | Astronomy |
| MAGIC | Major Atmospheric Gamma-ray Imaging Cherenkov (telescope) | - | Astronomy |
| MAGNETAR | Metaprogrammable AGent NETwork ARchitecture | - | Astronomy |
| MAGTF | Marine Air-Ground Task Force | - | Astronomy |
| MAH | Marshallese Language (ISO 639-2 Code) | - | Astronomy |
| MAL | Malayalam Language (ISO 639-2 Code) | - | Astronomy |
| MALD | Miniature Air-Launched Decoy | - | Astronomy |
| MANPADS | MAN-Portable Air Defence System | - | Astronomy |
| MANPRINT | MANpower And PeRsonnel INTegration | - | Astronomy |
| MAO | Mol?t? Astronomijos Observatorija (Moletai Astronomical Observatory) - MonoAmine Oxidase | - | Astronomy |
| MAPS | Monitoring Avian Productivity And Survivorship | - | Astronomy |
| MAQS | Museum Of The American Quilter's Society (max) | - | Astronomy |
| MAR | Marathi Language (ISO 639-2 Code) | - | Astronomy |
| MAR | Morocco (ISO 3166 Trigram) | - | Astronomy |
| MARBEF | Marine Biodiversity And Ecosystem Functioning | - | Astronomy |
| MARC | MAchine Readable Cataloging (often Also Defined As MAchine Readable Code) | - | Astronomy |
| MARCENT | United States Marine Forces Central Command | - | Astronomy |
| MARFORCC | Marine Force Component Commander | - | Astronomy |
| MAS | NATO Military Agency For Standardisation | - | Astronomy |
| MASAS | Monte Agliale Supernovae And Asteroid Survey | - | Astronomy |
| MASER | Microwave Amplification By Stimulated Emission Of Radiation | - | Astronomy |
| MASH | Mobile Army Surgical Hospital | - | Astronomy |
| MASINT | Measurement And Signature Intelligence | - | Astronomy |
| MATREX | Modeling Architecture For Technology | - | Astronomy |
| MATS | U.S. Military Air Transport Service (1948-1966) | - | Astronomy |
| MAVS | Mobile Artillery Vehicle System | - | Astronomy |
| MAX | Meta-reasoning Architecture For X | - | Astronomy |
| MB | Manitoba (postal Symbol) - Martinique (FIPS 10-4 Territory Code) | - | Astronomy |
| MBA | Main Battle Area - Main Belt Asteroid - Masters Of Business Administration | - | Astronomy |
| MBBL | (U.S. Armor Center) Mounted Battlespace Battle Laboratory | - | Astronomy |
| MBCC | Migratory Bird Conservation Commission (U.S.) | - | Astronomy |
| MBCS | Multi-Byte Character Set | - | Astronomy |
| MBE | Member Of The Order Of The British Empire | - | Astronomy |
| MBIDST | Member Of The British Institute Of Dental And Surgical Technologists | - | Astronomy |
| MBM | Meat And Bone Meal | - | Astronomy |
| MBT | Main Battle Tank | - | Astronomy |
| MBTA | Massachusetts Bay Transportation Authority - Migratory Bird Treaty Act (U.S.) | - | Astronomy |
| MBTI | Myers-Briggs Type Indicator | - | Astronomy |
| MC | Macao (FIPS 10-4 Territory Code) -Master Of Ceremonies -Megacoulomb -Military Cross (bravery Award) -Monaco (ISO 3166 Digram) -Mortar Carrier -Eleven Hundred (in Roman Numerals) | - | Astronomy |
| MC | Millicoulomb | - | Astronomy |
| MC&G | Mapping | - | Astronomy |
| MCA | Micro Channel Architecture - MultiChannel Analyzer | - | Astronomy |
| MCC | Marylebone Cricket Club - Mission Control Centre - Movement Control Centre | - | Astronomy |
| MCD | Minimal Cerebral Dysfunction | - | Astronomy |
| MCDL | Model Composition Definition Language | - | Astronomy |
| MCI | Microwave Communications | - | Astronomy |
| MCIS | Mellon Collie And The Infinite Sadness | - | Astronomy |
| MCM | Mine Counter-Measures | - | Astronomy |
| MCMV | Mine Counter-Measure Vessel | - | Astronomy |
| MCO | Monaco (ISO 3166 Trigram) -Movement Control Officer -Major Combat Operation | - | Astronomy |
| MCP | Maintenance Collection Point - Male Chauvinist Pig | - | Astronomy |
| MCR | My Chemical Romance | - | Astronomy |
| MCSF | Marine Corps Security Forces | - | Astronomy |
| MCT | Movement Control Team | - | Astronomy |
| MCTIS | Mounted Co-operative Target Identification System | - | Astronomy |
| MD | Fifteen Hundred (in Roman Numerals) - Maryland (postal Symbol) -Medicinae Doctor (Doctor Of Medicine/Medical Doctor) - Mentioned In Dispatches (military) -Moldova (FIPS 10-4 Country Code; ISO 3166 Digram) | - | Astronomy |
| MD | Mendelevium | - | Astronomy |
| MDA | Main Defensive Action/Area -Moldova (ISO 3166 Trigram) - Muscular Dystrophy Association | - | Astronomy |
| MDF | Main Distribution Frame - Medium-Density Fibreboard | - | Astronomy |
| MDG | Madagascar (ISO 3166 Trigram) -UN Millennium Development Goal | - | Astronomy |
| MDL | Moldovan Leu (ISO 4217 Currency Code) | - | Astronomy |
| MDM | Michigan Dartmouth MIT Telescope Consortium | - | Astronomy |
| MDMA | MethyleneDioxy-MethylAmphetamine (also Known As Ecstasy) | - | Astronomy |
| MDO | Membership Driven Organisation | - | Astronomy |
| MDP | Markov Decision Process | - | Astronomy |
| MDT | Mountain Daylight Saving Time (UTC-6 Hours) | - | Astronomy |
| MDV | Maldives (ISO 3166 Trigram) | - | Astronomy |
| MDW | Chicago Midway International Airport In Chicago | - | Astronomy |
| ME | Maine (postal Symbol) -Mechanical Engineer[ing] - Medical Examiner - Middle East -Middle East Airlines (IATA Code) -Memory Effect - Myalgic Encephalomyelitis | - | Astronomy |
| ME | Messerschmitt | - | Astronomy |
| MEA | Maine Educational Assessment - Middle East Airlines - Millennium Ecosystem Assessment - Minimum Enroute Altitude - Mono-ethanol Amine - Multilateral Environmental Agreement | - | Astronomy |
| MEC | Maine Central Railroad (AAR Reporting Mark) - Manta | - | Astronomy |
| MECC | Minnesota Educational Computing Consortium (later Corporation | - | Astronomy |
| MECO | Main Engine Cutoff (NASA Code For Point In Space Shuttle Assent When Three Main Engines Are Shut Down Prior To External Tank (ET) Separation) | - | Astronomy |
| MEDEVAC | Medical Evacuation | - | Astronomy |
| MEDS | Mission Essential Data Set | - | Astronomy |
| MEF | U.S. Marine Expeditionary Force - Minimum Essential Facility - Mission Essential Force | - | Astronomy |
| MEG | MagnetoEncephaloGraphy | - | Astronomy |
| MEI | Matsushita Electric Industrial Co. | - | Astronomy |
| MEK | Magyar Elektronikus Konyvtar - Message Encryption Key - Methyl Ethyl Ketone -Mujahideen Al-Khalq | - | Astronomy |
| MEL | Maya Embedded Language - Michigan Electronic Library | - | Astronomy |
| MEM | Message Exchange Mechanism -Microelectromechanical | - | Astronomy |
| MEMS | Microelectromechanical System | - | Astronomy |
| MEO | Medium Earth Orbit - Most Efficient Organization | - | Astronomy |
| MEP | Mechanical Electrical Plumbing - Member Of The European Parliament - Missions ?trang?res De Paris (French For Paris Foreign Missions) - Model European Parliament | - | Astronomy |
| MER | Mars Exploration Rover | - | Astronomy |
