| Acronym | Definition |
|---|
| SI | Sports Illustrated |
| SI | Slovenia (international code) |
| SI | Silicon |
| SI | Systems Integration |
| SI | Système International d'Unités (international system of units based on the Meter, Kilogram, Second, Ampere, Kelvin, Candela, and Mole) |
| SI | System Information (Norton Utilities) |
| SI | System Information |
| SI | Statutory Instrument (UK Legislation term) |
| SI | Service Information |
| SI | School of Information (university) |
| SI | Self-Injury |
| SI | Square Inch(es) |
| SI | Smithsonian Institution |
| SI | Supplemental Instruction |
| SI | Signal Integrity |
| SI | Spark Ignition |
| SI | Shift In |
| SI | Slightly Included (second lowest quality of diamond) |
| SI | Serious Injury |
| SI | International System of Units (metric system; more commonly seen as ISU) |
| SI | South Island (New Zealand) |
| SI | Staten Island |
| SI | Sistema Informativo |
| SI | Structural Integrity |
| SI | Systèmes d'Information |
| SI | Standardization (Flight) Instructor |
| SI | System International |
| SI | Site Investigation |
| SI | System Integrator |
| SI | Superimpose |
| SI | Sensory Integration |
| SI | Sexual Intercourse |
| SI | System Interface |
| SI | Sports Interactive (game) |
| SI | Still Image |
| SI | Shut-In |
| SI | Small Intestine |
| SI | Seriously Ill |
| SI | Self Inflicted |
| SI | Scientific Investigation |
| SI | Sistema de Informação |
| SI | Systems Integrator |
| SI | Sacroiliac |
| SI | Singapore Idol (Singapore TV show) |
| SI | Study Island (software) |
| SI | Strategic Initiative (management) |
| SI | Self-Insert (fiction) |
| SI | Software Item |
| SI | Simply Irresistible (song) |
| SI | Speaker Independent |
| SI | Self-Incompatibility (botany) |
| SI | Science and Industry |
| SI | Sacroiliac Joint |
| SI | Shadow Image |
| SI | Siena, Toscana (Italian province) |
| SI | Swine Influenza |
| SI | Site Inspection |
| SI | Systems Intelligence (psychological theory) |
| SI | Swap-In |
| SI | Soroptimist International |
| SI | Sub Inspector |
| SI | Simple Interest |
| SI | Synergos Institute (San Diego, CA) |
| SI | Softice (debugger) |
| SI | Service Index |
| SI | Swarm Intelligence |
| SI | Sudden Impact (movie) |
| SI | Source Index |
| SI | Suicidal Ideation |
| SI | Scientific Instrument |
| SI | Sensitive Item (algorithm) |
| SI | Step-Index |
| SI | Service Indicator (SS7 SCCP) |
| SI | Situationist International (governing body of political dissidents circa 1968) |
| SI | Specialty Item |
| SI | SPARC International, Inc. |
| SI | Service Integrator |
| SI | Semiconductor International |
| SI | Shared Interest (New York, NY) |
| SI | Side Information |
| SI | Special Instruction |
| SI | Schweizer Informatiker Gesellschaft (Zurich, Switzerland) |
| SI | Shrouded Isles (gaming, Dark Age of Camelot add-on) |
| SI | Silicon Image, Inc |
| SI | Specific Impulse (measure of efficiency of rocket engines) |
| SI | Surface Impoundment |
| SI | Special Intelligence |
| SI | Special Inspection |
| SI | Simplified Issue (life insurance) |
| SI | Saturation Index |
| SI | Signal Interference |
| SI | Science Instrument (Hubble Telescope) |
| SI | Soil Investigation |
| SI | Similarity Index |
| SI | Strategic Investor |
| SI | Speech Impairment |
| SI | Shift-Invariant |
| SI | Salt Institute |
| SI | Solution Implementation |
| SI | Summons Issued |
| SI | Sarcasm Intended |
| SI | Systematic Innovation |
| SI | Sailing Instruction |
| SI | Security Inspector |
| SI | Solving International |
| SI | Strange Interlude (Eugene O'Neill book) |
| SI | Sensitivity Information |
| SI | Self-Insertion (fan fiction) |
| SI | Shipping Instruction |
| SI | Siilinjärvi (Finnish) |
| SI | Sexually Inappropriate |
| SI | Secondary Injection |
| SI | Solar Influences |
| SI | Scalable Infrastructure |
| SI | Skill Identifier (US Army) |
| SI | Sucrase-Isomaltase Deficiency |
| SI | Settlement Instruction |
| SI | Surveillance and Intelligence |
| SI | Self Isolation |
| SI | Safeguards Information |
| SI | Safe Intake |
| SI | Standardization and Interoperability |
| SI | Spiritual Intervention (religious doctrine) |
| SI | Surveillance Index |
| SI | Straight-In Approach |
| SI | Spray Injection |
| SI | Sergeant Instructor |
| SI | Saint Ignatius College Preparatory (San Francisco, California) |
| SI | Smoothness Index |
| SI | Society for India (Cornell University) |
| SI | Summary Investigation |
| SI | Scintillation Index |
| SI | Supporting Installation |
| SI | Samahang Ilokano (Filipino college organization) |
| SI | Solar Inertial |
| SI | Supplementary Insulation |
| SI | Sponsor Identification |
| SI | Surveyor Intern |
| SI | Sport Injected (Honda) |
| SI | Simple Interference |
| SI | Sector Iridectomy (partial removal of the iris of the eye) |
| SI | Septuagint Institute (Trinity Western University, Canada) |
| SI | Selective Interchange |
| SI | Semiconductor Ignition |
| SI | Safety/Site Inspection |
| SI | Shadow Instruction (computer instruction set) |
| SI | Sharma Industries |
| SI | Somatosensory Receiving Area (in parietal lobe of cortex) |
| SI | Spokane International Railroad Company |
| SI | Signaling Intelligence |
| SI | Normalized Subthreshold Swing |
| SI | Sippy International |
| SI | Slave Illuminator |
| SI | Systems Industries, Incorporated |
| SI | Societatis Iesu, Latin for Society of Jesus, Jesuits (religious order) |
| SI | Secretaría Indigenista (Spanish: Indigenous Secertariat) |