| Acronym | Meaning |
| SIEC | Spherical Inertial Electrostatic Confinement |
| SIFCA | Slurry-infiltrated Fiber Castable [ceramic Casting Technique] |
| SIFS | Solvent-induced Frequency Shifts |
| SIFT | Selected Ion Flow Tube |
| SIG | Special Interest Group |
| SIGLE | System For Information On Grey Literature In Europe |
| SIIA | Software & Information Industry Association |
| SIIMS | Secondary Ion Imaging Mass Spectrometry |
| SILAC | Stable Isotope Labeling By Amino Acids In Cell Culture |
| SILAR | Successive Ionic Layer Adsorption And Reaction |
| SILC | Stress-induced Leakage Current |
| SILO | Strain Induced Lateral Ordering |
| SIM | Selected Ion Monitoring |
| SIM | Standards Injection Module |
| SIM | Stress-induced Martensite |
| SIMAAC | Simultaneous Multi-element Analysis With A Continuum Source |
| SIMBA | Selective Inverse Multiple Bond Analysis |
| SIMCA | Soft Independent Modeling Of Class Analogy (principal Component Analysis) |
| SIMD | Single-instruction Multiple-data |
| SIMM | Single Inline Memory Module |
| SIMOX | Separation By Implanted Oxygen |
| SIMP | Solar Irradiance Monitoring Program |
| SIMPEDS | Safety In Mines Personal Dust Sampler |
| SIMPLISMA | Simple-to-use Interactive Self-modeling Mixture Analysis |
| SIMR | Swedish Institute For Metals Research |
| SIMS | Secondary Ion Mass Spectrometry |
| SIMS | Surface Impoundment Modeling System |
| SINDO | Scaled Intermediate Neglect Of Differential Overlap |
| SINDO | Symmetrically Orthogonalized Intermediate Neglect Of Differential Overlap |
| SINDO/F | SINDO Free Of Empirical Parameters |
| SINGER | System-wide Information Network For Genetic Resources |
| SINK | Saturation Internuclear Kinetics |
| SINNMR | Sonically Induced Narrowing Of The NMR Spectra Of Solids |
| SIOA | Surface-induced Optical Anisotropy |
| SIOS | Semiclassical Infinite-order Sudden (in Rotationally-inelastic Scattering Calcns.) |
| SIP | State Implementation Plan |
| SIPOS | Semi-insulating Polycrystalline Silicon |
| SIPP | Sodium Iron Pyrophosphatepar |
| SIR | Selected Ion Recording |
| SIR | Statutory Invention Registration |
| SIR-C | Spaceborne Imaging Radar-C |
| SIRIS | Sputter-initiated Resonance Ionization Spectroscopy |
| SIRM | Scanning Infrared Microscope |
| SIRMO | Senior Information Resource Management Officer |
| SIRMS | Stable Isotope Ratio Mass Spectrometry |
| SIRNA | Small Interfering RNA |
| SIRS | Systemic Inflammatory Response Syndrome |
| SIRTF | Space Infrared Telescope Facility |
| SIS | Safety Injection System |
| SIS | Stepwise Isothermal Segregation |