| n | Nano |
| N | Nitrogen |
| Na | Sodium |
| NA | Numerical aperture |
| NAA | Neutron activation analysis |
| NADCAP | National Aerospace and Defense Contractors Accreditation Procedures |
| NAND | Logic term - Not-AND |
| nano | 1.00E-9 (1/1,000,000,000; symbolized by the prefix n) |
| NASA | National Aeronautics and Space Administration |
| Nb | Niobium |
| NBE | Near Band Edge |
| NBR | Nitrile Butadiene-Acrylonitrile Rubber |
| NBS | National Bureau of Standards |
| NC | NanoCrystal; Numerical control |
| NCMS | National Center for Manufacturing and Science |
| NCS | Network Communication Standard |
| NCTA | National Cable & Telecommunications Association |
| Nd | Neodymium |
| ND | New Donor; Native defect |
| NDA | Nondisclosure agreement |
| NDE | Nondestructive evaluation |
| NDIR | Non-dispersive infrared spectroscopy |
| NDP | Neutron depth profiling |
| NDT | Nondestructive testing |
| NDUV | Non-dispersive ultraviolet spectroscopy |
| Ne | Neon |
| NEC | National Electric Code |
| NECQ | National Electronics Component Qualification System |
| NEMA | National Electrical Manufacturers Association |
| NEP | Noise Equivalent Power |
| NESHAP | National emissions standards for hazardous air pollutants |
| NETD | Noise Equivalent Temperature Difference |
| NewsML | News Mark-up Language |
| NEXAFS | Near Edge X-ray Absorption Fine Structure |
| NEXT | Near End Crosstalk |
| NFE | Near Free Electron |
| NFOM | Near field optical microscopy |
| NFPA | National Fire Protection Association |
| Ni | Nickel |
| NIC | Network Interface Card |
| NIH | National Institutes of Health |
| NIST | National Institute for Science and Technology |
| NITF | News Industry Text Format |
| NIU | Network Interface Unit |
| NLO | Non-Linear Optic |
| NLOVS | Niet Lineaire Optica Vaste Stoffen |
| NLR | Non-Linear Refraction |
| nm | Nanometer |
| NMOS | Negative channel metal-oxide semiconductor |
| NMR | Nuclear Magnetic Resonance; Normal mode rejection |
| NN | Nearest Neighbor; Neural network |
| NNN | Next Nearest Neighbor |
| No | Nobelium |
| NOB | Non-biased Optical Bistable (device) |
| NOCI | Non Orthogonal Configuration Interaction |
| O | Oxygen |
| OA | Organic Acid |
| OAAF | Open Advanced Authoring Format |
| OAC | Optical Absorption Coefficient |
| OASIS | Organization for the Advancement of Structured Information Standards |
| OBA | Object Behavior Analysis |
| OBEM | Object-Based Equipment Model |
| OBIC | Optical Beam Induced Current |
| OBL | Object-Based Language |
| OC | Open Collector |
| OCAP | OpenCable Application Platform |
| OCR | Optical Character Recognition |
| OCSP | Online Certificate Status Protocol |
| OD | Outside Diameter |
| O-D | Zero dimensional |
| ODBC | Open DataBase Connectivity |
| ODEND | OR Optical Detected ENDOR |
| ODEPR | Optical Detected EPR |
| ODMII | Optical Detected Microwave induced Impact Ionization |
| ODMR | Optical Detected Magnetic Resonance |
| ODP | Ozone Depletion Potential |
| ODR | Oscillating Disk Rheometer |
| ODRL | Open Digital Rights Language |
| ODS | Ozone-Depleting Substances |
| OEBR | Optical Edge Bead Removal |
| OEE | Overall Equipment Effectiveness |
| OEIC | OptoElectronic IC |
| OEM | Original Equipment Manufacturer; Office of Emergency Management |
| OES | Oti l E i i S t |
| OES | Optical Emission Spectroscopy |
| OFDM | Orthogonal Frequency Division Multiplexing |
| OFHC | Oxygen-Free High-Conductivity Copper |
| OFS | Open File System |
| OI | Opto Isolator |
| OID | Object Interaction Diagram; Object IDentification |
| OIL | Ontology Interface Layer |
| OIM | Orientational Imaging Microscopy |
| OL | Overlay |
| OLB | Outer Lead Bond |
| OLE | Object Linking and Embedding |
| OLED | Organic LED |
| OLS | Ordinary Least Squares |
| OM | Operational Modeling; Optical Microscopy; OrganoMetallic |
| OMA | Object Management Architecture; Open Mobile Alliance |
| OMG | Object Management Group |
| OMS | Optical Mass Spectroscopy |
| OMT | Object Modeling Technique |
| OMVPE | Organometallic VPE |
| ONE | Open Network Environment (Sun) |
| ONP | Optical Nuclear Polarization |
| OO | Object-Oriented |
| OOA | Object-Oriented Analysis |
| OOD | Object-Oriented Design |
| OODB | Object-Oriented Data Base |
| OODBMS | Object-Oriented Data Base Management System |
| OOP | Object Oriented Programming |
| OOPS | Object-Oriented Programming System |
| OpAmp | Operational Amplifier |
| OPC | Optical Particle Counter; Optical Proximity Correction |
| OPL | Optically Pumped Laser |
| OPSE | Optically Pumped Stimulated Emission |
| OPW | Orthogonalized Plane Wave |
| OR | Logic term - OR |
| ORB | Object Request Broker |
| ORM | Other Regulated Material |
| ORNL | Oak Ridge National Laboratory |
| Os | Osmium |
| OS | Operating System |
| OSC | Organic Solderability Coating |
| OSCI | Online Services Computer Interface |
| OSD | On Screen Display |
| OSDH | Optical Shubnikhov-de Haas effect |
| OSF | Open Systems Foundation |
| OSHA | Occupational Safety Hazards Act |
| OSI | Open System Interconnection |
| OSP | Organic Solderability Preservative |
| OSRM | Office of Standard Reference Materials |
| OSS | Object Services Standard; Operation Support System |
| OSV | Offset Scan Voting |
| OTP | One Time Programmable |
| OTPROM | One Time Programmable ROM |
| OVD | Outside Vapor Deposition ( Corning ) |
| OVL | Overlay |
| OX | OXide |