| Teachers Summer Institute | Academic & Science |
| Technical School of Innovation | Academic & Science |
| Total Solar Irradiance | Academic & Science |
| Tsili Tsili | Airport Code |
| TownHall Software Inc | Business |
| Transformed Services Inc. | Business |
| Transformation Systems Inc | Business |
| Town Sports International | Business |
| Total Safety Inc | Business |
| Trinkle Sales Inc | Business |
| Transit Systems Inc | Business |
| The Steel Index | Business |
| Timber Stand Improvement | Community |
| Travel Solutions International | Community |
| Transmitting Subscriber Identification | Computer and Networking |
| Technical Specifications for Interoperability | Computer Hardware |
| Telecommunication Security Information | Computing |
| Technology Service and Integrity | Computing |
| Technology Systems Institution | Computing |
| Technology Services Innovation | Computing |
| Transmission Station Identification | Computing |
| Trigger Supervisor Interface | Computing |
| Termination System Information | Computing |
| Transport Service Interpreter | Computing |
| Transmitting Subscriber Information | Computing |
| Two Symbol Interval | Computing |
| Time Slice Intervals | Computing |
| Tom Sawyer Island | Country Specific |
| Torres Strait Islander | Country Specific |
| Transmitting Station Identifier | General |
| Time Slot Interchanger | General |
| Timber And Soil Improvement | General |
| The Systematic Inventory | General |
| Total Scene Irradiance | General |
| Texas Success Initiative | General |
| tal Solar Irradiance | General |
| Turbo Stratified Injection | General |
| Trick Scooters International | General |
| Thyroid Stimulating Immunoglobulions | General |
| Technical Support Internship | General |
| True Strength Index | General |
| Triple Sugar Iron | General |
| Thermal System Insulation | General |
| Tal Scene Irradiance | General |
| Twincharger Stratified Injection | General |
| Turbo Supercharged Injection | General |
| Twin Spark Ignition | General |
| Traffic Sign Identification | General |
| Triple Serial Interface | General |
| Transport Session Identifier | General |
| Twin Swrill Injection | General |
| Technical Sales International | General |
| Twin Stratified Injection | General |
| Tissue Synchronization Imaging | General |
| Turbocharged Stratified Injection | General |
| Transactional Strategies Instruction | General |
| tal Skill Index | General |
| TCW Strategic Income | General |
| Turn Signal Indicators | General |
| Turbo Sport Induction | General |
| Tuned Sport Injection | General |
| Turbo Charged Spark Ignition | General |
| Technical Service Information | General |
| Traffic Safety Institute | General |
| Technical System for Interoperability | General |
| Touring Super Injection | General |
| Truck Service Inc. | General |
| TRADOC (TRAining and DOCtrine Command) Simulation Internet | Governmental |
| Transportation Safety Institute | Governmental |
| Two Speed Idle | Governmental |
| Turbo Sport Intercooled | Governmental |
| Turbo Sport Issue | Governmental |
| Turkish Statistical Institute | Governmental |
| Transportation Services Index | Governmental |
| Traumatic Stress Institute | Governmental |
| Turn Signal Intakes | Governmental |
| The Sulphur Institute | Governmental |
| Teaching Summer Institute | Governmental |
| Trading Standards Institute | Governmental |
| TENKASI JN | Indian Railway Station |
| TüRkiye Saati Ile | International |
| Thyroid Stimulating Immunoglobulin | Medical |
| Tradoc Simulation Internet | Military and Defence |
| The Silent Interrogators | Military and Defence |
| Tsimshian | Regional |
| Tsili Tsili, Papua New Guinea | Regional |
| Trophic State Index | Regional |
| Third Season Inconsistency | Sports |
| Total Skill Index | Sports & Games |
| Telecommunications Services Inc | Telecommunication |
| Time Slot Interchange | Telecommunication |