| Acronym | Definition |
|---|
| TSI | Total Solar Irradiance (sum solar light in energy per unit of time) |
| TSI | Trading Standards Institute (UK) |
| TSI | Timber Stand Improvement (forestry) |
| TSI | Transportation Safety Institute (US DOT) |
| TSI | Technical Specifications for Interoperability (railway standards) |
| TSI | Thyroid Stimulating Immunoglobulin |
| TSI | Triple Sugar Iron |
| TSI | Thermal System Insulation (industry & regulatory term used with regard to asbestos) |
| TSI | Transportation Security Incident |
| TSI | Turbine Supervisory Instrumentation |
| TSI | Treehouse Software Inc. |
| TSI | Telecommunications Service Inquiry (Australia) |
| TSI | Trophic Status Index (water quality) |
| TSI | Transmitting Subscriber Identification |
| TSI | Traumatic Stress Institute |
| TSI | Transportation Security Inspector (US DHS) |
| TSI | Thermal Systems Inc (Scarborough, Maine) |
| TSI | Total Sum Insured |
| TSI | Tech Services International |
| TSI | Turbo Stratified Injection |
| TSI | Technical Service Information |
| TSI | Transportation Systems Institute |
| TSI | Teach Services Inc. (book publisher) |
| TSI | Twin Stage Impinger |
| TSI | Talent Scouts Inc. |
| TSI | Total Systems Integration, Inc (Galion, Ohio) |
| TSI | True Sales Initiative |
| TSI | Thinking Style Inventory |
| TSI | The Skeptical Inquirer |
| TSI | Transportation System Improvement |
| TSI | Technical Sales International, LLC (Austin, TX ) |
| TSI | Technical Standardization Inspection |
| TSI | Territory Sales Incharge |
| TSI | Technical Site Information |
| TSI | Time-Space Illusion |
| TSI | Turksportal Interactive |
| TSI | Turbo Sport Injection (automotive) |
| TSI | Tactical Situation Indicator |
| TSI | Training Systems Integration (US Army) |
| TSI | Temporal Sharing Interface |
| TSI | Time Slot Interchange/r |
| TSI | Transmitting Station Information (Fax) |
| TSI | Transport Subsystem Interface |
| TSI | TEMPEST Security Index |
| TSI | Typography Systems International |
| TSI | Threshold Signal-to-Interference Ratio |
| TSI | Terminal Support Interface |
| TSI | Training Support Infrastructure (US Army TRADOC/ATSC/ATMD) |
| TSI | Test Ship Integration |
| TSI | TRADOC Simulation Internet |
| TSI | Training Standards Institute |
| TSI | Target Status Indicator |
| TSI | Top Systems Inc. |
| TSI | Terminal Signaling Interface |
| TSI | That's Scrap Inc (Apple Valley, MN) |
| TSI | Touring Sport Injection |