| m | Meter; Molar |
| MAA | Monomethylarsenic Acid |
| MABS | Methyl Methacrylate Acrylonitrile Butadiene Styrene Resin; Monoclonal Antibodies |
| MAC | Multi Agency Contract; Multiple Award Contract |
| MACRODATA | AGGREGATED DATA; SUMMARIZED DATA |
| MACT | Maximum Achievable Control Technology; Maximum Available Control Technology |
| MAHs | Monocyclic Aromatic Hydrocarbons |
| Major HAP Facility | Major Hazardous Air Pollutant Facility |
| Major PSD Modification | Major Prevention of Serious Deterioration Modification |
| Major PSD Stationary Source | Major Prevention of Serious Deterioration Modification Stationary Source |
| MALDI | Matrix-Assisted Laser Desorption Ionization Mass Spectrometry |
| MALDI-ToF Mass Spectrometry | Matrix Assisted Laser Desorption/Ionization Time-of-Flight Mass Spectrometry |
| MARAMA | Mid-Atlantic Regional Air Management Association |
| Marine SI Engine Eligible Sale | Marine Spark Ignition Engine Eligible Sale |
| MATC | Maximum Acceptable Toxic Concentration |
| Material Containing HAP | Material Containing Hazardous Air Pollutants |
| Material Containing MFHAP | Material Containing Metal Fabrication and Finishing Hazardous Air Pollutants |
| Material P&D | Material Processing and Delivery |
| MAUT | Multi-Attribute Utility Theory |
| Maximum Potential NOX Emission Rate | Maximum Potential Nitrogen Oxides Emission Rate |
| Maximum Rated RPM | Maximum Rated Revolutions per Minute |
| MB | Mass Balance |
| MBAS | Methylene-Blue-Active Substances |
| MBE | Minority Business Enterprise |
| MBF | Matrix-Based Fertilizer |
| MBFs | matrix based fertilizers |
| MBL | Marine Boundary Layer |
| MBOCA | 4,4'-Methylenebis(2-chloroaniline) |
| MBS | Methyl Methacrylate Butadiene Styrene Resin |
| MC | microcystin; Military Constituents |
| mCA | Microcantilever Array; Monte Carlo Analysis |
| MCAN | Microbial Commercial Activity Notice |
| MCC | Mesoscale Convective Complex |
| MCE | Maximum Credible Earthquake; Mixed cellulose ester |
| MCHM | 4-Methylcyclohexanemethanol |
| MCL | Maximum Contaminant Level (regulatory) |
| MCLG | Maximum Contaminant Level Goal (regulatory) |
| MCLGs | Maximum Contaminant Goals |
| MCLs | Maximum Contaminant Levels |
| MCS | Multiple Chemical Sensitivity |
| MCV | Mesoscale Connective Vortex |
| MD | Mineral Dust |
| MDC | Maximum Detected Concentration |
| MDE | Maximum Design Earthquake |
| MDF | Medium-Density Fiberboard |
| MDL | Maximum Daily Effluent Limitation; Method Detection Limit |
| MDPV | Medium-Duty Passenger Vehicle |
| Medium CAFO | Medium Concentrated Animal Feeding Operation |
| Medium HMIWI | Medium Hospital/Medical/Infectious Waste Incinerator |
| MEE | Microexposure Event Analysis |
| MEI | maximum exposed individual |
| MERV | Minimum Efficiency Reporting Value |
| MET | Metabolic Equivalent of Work |
| Metal Oxides | Smelting |
| Metallic HAP | Metallic Hazardous Air Pollutant |
| Metallic HAP Process Vent | Metallic Hazardous Air Pollutant Process Vent |
| Methoxychlor | Certain Pesticide |
| MF | Modifying Factor |
| MFCD | Mechanical Flow Control Device |
| MFFSR | Multifilter Rotating Shadowband Radiometer |
| MFHAP | Metal Fabrication and Finishing Hazardous Air Pollutant |
| MFO | Mixed-Function Oxidase |
| mg | Milligram |
| Mg(2+) | magnesium |
| mgd | Million Gallons per Day |
| MGE | Mobile Genetic Element |
| mg/L | Milligrams per Liter; milligrams/liter |
| MHHW | mean higher high water |
| MHW | mean high water |
| MICROCOPY | MICROPHOTOGRAPH |
| MICRODATA | UNAGGREGATED DATA; UNSUMMARIZED DATA |
| MID | minimum infective dose |
| MIESR | Matrix Isolation Electron Spin Resonance |
| Milling | Machining |
| min | Minimum |
| Minimum 96-Hour LC5 | Minimum 96-Hour Lethal Concentration 50 Percent |
| Minimum HCl Sorbent Flow Rate | Minimum Hydrogen Chloride Sorbent Flow Rate |
| MIP | Maximum Injection Pressure |
| MIPS | Material Input per Unit Service |
| MIR | maximum individual risk |
| MIS | Management Information System |
| Miscellaneous ACM | Miscellaneous Asbestos-Containing Material |
| MISR | Multi-Angle Imaging Spectroradiometer |
| ML | Minimum Level |
| ML Method | Maximum Likelihood Method |
| MLE | Maximum Likelihood Estimate |
| MLO | Mycoplasma-Like Organism |
| MM | Major Modification |
| MM5 | Mesoscale Model |
| MMAD | Mass Median Aerodynamic Diameter |
| mm/Hg | Millimeters of Mercury |
| MMR | Mismatch Repair |
| MMRP | Military Munitions Response Program |
| MNA | Monitored Natural Attenuation |
| MOA | mechanism of action; Memorandum of Agreement |
| Modified HMIWI | Modified Hospital/Medical/Infectious Waste Incinerator |
| Modular Excess Air MWC | Modular Excess-Air Municipal Waste Combustor |
| Modular Starved Air MWC | Modular Starved-Air Municipal Waste Combustor |
| MOE | Margin of Exposure |
| Molten Metal | Melting Operation |
| Monsoon Circulation | Planetary Wind Belt |
| MOR Process | Metal Oxygen Refining Process |
| MOS | Margin of Safety |
| MOU | Memorandum of Understanding |
| MOVES | Motor Vehicle Emission Simulator |
| MP | Manufacturing Use Product; Montreal Protocol |
| MPA | Marine Protected Area |
| MPA (Monitoring) | Monitoring Planning Area |
| MPCD | Marine Pollution Control Device |
| MPG | Miles Per Gallon |
| MPN | Most Probable Number |
| MPO | Metropolitan Planning Organization |
| MQOs | Measurement Quality Objectives |
| MRDL | Maximum Residual Disinfectant Level (regulatory) |
| MRDLG | Maximum Residual Disinfectant Level Goal (regulatory) |
| mrem | Millirem |
| MRF | Material Recovery Facility; Materials Recovery Facility |
| MRID | Master Record Identification (number) |
| MRL | Minimum Reporting Level (regulatory) |
| mRNA | Messenger RNA |
| MS | Major Source; Mass Spectrometer; Mass Spectrometry; Matrix Spike; Modified Source |
| MS and MSD | Matrix Spike and Matrix Spike Duplicate |
| MS4 | Municipal Separate Storm Sewer System |
| MSA | Metropolitan Statistical Area |
| MSDS | Material Safety Data Sheet |
| MSE | Mobile Source Emissions |
| MSGP | Multi-Sector General Permit |
| MSL | mean sea level |
| MS-MS | Tandem Mass Spectrometry |
| MSR | Management Systems Review |
| MSS | Major Stationary Source |
| MST | Microbial Source Tracking |
| MSW | Municipal Solid Waste |
| MSW Landfill | Municipal Solid Waste Landfill |
| MSW Landfill Emission | Municipal Solid Waste Landfill Emission |
| MSW Landfill Federal Plan | Municipal Solid Waste Landfill Federal Plan |
| MSWLF Structural Component | Municipal Solid Waste Landfill Structural Component |
| MSWLF Unit | Municipal Solid Waste Landfill Unit |
| MSX | Multinucleated Sphere Unknown |
| MTBE | Methyl tertiary-Butyl Ether |
| MTBE Groundwater Contamination | Methyl Tertiary Butyl Ether Groundwater Contamination |
| MTBE Removal | Methyl Tertiary Butyl Ether Removal |
| MTCIU | Middle-Tier Categorical Industrial User |
| MTD | Maximum Tolerated Dose |
| mtDNA | Mitochondrial DNA |
| MTEC | Maximum Theoretical Emissions Concentration |
| MTS | Marine Transportation System |
| MTTP | Maximum Total Trihalomethane Potential |
| Multiple MOA | Multiple Modes of Action |
| MUTA | Mutagenicity |
| MVA | Multivariate Analysis |
| MVAC-Like Appliance | Motor Vehicle Air Conditioner-Like Appliance |
| MVAC-Like Appliances | Motor Vehicle Air Conditioner-Like Appliances |
| mVOC | Microbial Volatile Organic Compound |
| MVWDW | Motor Vehicle Waste Disposal Well |
| MW | megawatt; molecular weight |
| MWC | Municipal Waste Combustor |
| MWC Acid Gas | Municipal Waste Combustor Acid Gas |
| MWC Metals | Municipal Waste Combustor Metals |
| MWC Organics | Municipal Waste Combustor Organics |
| MWCNT | Multi-Walled Carbon Nanotube |
| MY | Model Year |