| Acronym | Meaning |
| NIS | Neutron Inelastic-scattering |
| NIS | Sodium Iodide Symporter |
| NISO | National Information Standards Organization |
| NIST | National Institute Of Standards And Technology (formerly NBS) |
| NISTO | Non-integer Slater-type Orbital |
| NITEP | National Incinerator Testing And Evaluation Program (Canada) |
| NITFS | National Imagery Transmission Format Standard |
| NITM | Normalized Irreducible Tensorial Matrix |
| NITS | National Institute Of Technology And Standards |
| NIWA | National Institute For Water And Atm. Research [New Zealand] |
| NIWG | National Interagency Work Group |
| NIWR | National Institute For Water Research (South Africa) |
| NIXSW | Normal Incidence X-ray Standing Wave |
| NJTSR | New Jersey Trade Secret Registry |
| NK | Natural Killer |
| NKA | (Na2O + K2O)/Al2O3 |
| NKB | Nachklarbecken; Secondary Settling Tank |
| NKN | Sodium Potassium Niobate |
| NL-SCF | Nonlocal (density Functional)-self-consistent Field |
| NLAP | Nonlimited Aeration Process (for Waste Treatment) |
| NLC | Next Linear Collider |
| NLC | Nematic Liq. Crystal |
| NLC | Noctilucent Cloud |
| NLCR | Nonlinear Continuum Regression |
| NLDA | Nonlocal Density Approxn. |
| NLDF | Nonlocal Density Functional |
| NLDFT | Nonlocal Density Functional Theory |
| NLET | National Laboratory For Environmental Testing |
| NLGI | National Lubricating Grease Institute |
| NLLAP | National Lead Laboratory Accreditation Program |
| NLLS | Nonlinear Least Squares |
| NLLSCF | Nonlinear Least Squares Curve Fitting |
| NLLSF | Non-linear Least Square Fitting |
| NLLSQ | Nonlinear Least Squares |
| NLM | National Library Of Medicine |
| NLM | Nonlinear Mapping |
| NLMO | Natural Localized Mol. Orbital |
| NLMO | Nonorthogonal Localized Molecular Orbital |
| NLO | Nonlinear Optical |
| NLP | Natural Language Processing |
| NLP | Nonlinear Programming |
| NLPLS | Nonlinear Partial Least Squares |
| NLPS | Nonreactive |
| NLS | Nonlinear Schroedinger Equation |
| NLS | Nuclear Localization Signal |
| NLSD | Nonlocal Spin Density (functional Theory) |
| NLSLE | Nonlinear Schroedinger-Langevin Equation |
| NLTE | Non-local Thermodynamic Equilibrium |
| NLXC | Nonlocal Exchange Correlation (potential) |
| NMA | Nuclear Microanalysis |