| Acronym | Meaning |
| AST | Aboveground Storage Tank |
| AST | Aspartate Aminotransferase |
| ASTDR | Agency For Toxic Substances And Disease Registry |
| ASTER | Advanced Spaceborne Thermal Emission And Reflection Radiometer (EOS) |
| ASTER | Assessment Tools For The Evaluation Of Risk |
| ASTER | Acceptable Short-term Exposure Range |
| ASTEX | Atlantic Stratus Transition Experiment |
| ASTHO | Association Of State And Territorial Health Officers |
| ASTIS | Arctic Science And Technology Information System |
| ASTM | American Society For Testing And Materials |
| ASTRA | Alliance For Science And Technology Research In America |
| ASTRAP | Advanced Statistical Trajectory Regional Air Pollution Model |
| ASTROS | Automated Structural Optimization System |
| ASTSWMO | Association Of State And Territorial Solid Waste Management Officials |
| ASU | Air Separation Unit |
| ASV | Anodic Stripping Voltammetry |
| ASW | Acoustic Surface Wave Measurements |
| ASW | Amorphous Solid Water |
| ASW | Artificial Seawater |
| ASW | Augmented Spherical Wave |
| ASW | Axial Spin Wave (type Of Hartee-Fock Wave Function) |
| ASX | Aluminosilicate Xerogel |
| ATA | Anaerobic Toxicity Assay |
| ATA | Average T-matrix Approximation |
| ATAD | Auto Thermal Aerobic Digestion |
| ATAS | Advance Technology Alert System |
| ATBN | Amine Terminated Butadiene Acrylonitrile (rubber) |
| ATCA | Australia Telescope Compact Array |
| ATCC | American Type Culture Collection |
| ATCM | Air Toxics Control Measure |
| ATD | Alzheimer Type Dementia |
| ATD | Atmospheric Technology Division |
| ATDC | After Top Dead Center (gasoline And Diesel Engine Timing) |
| ATDHF | Adiabatic Time-dependent Hartree-Fock |
| ATE | Automatic Test Equipment |
| ATECT | Acetyl Triethyl Citrate |
| ATEM | Analytical Transmission Electron Microscopy |
| ATERIS | Air Toxics Exposure And Risk Information System |
| ATES | Aquifer Thermal Energy Storage |
| ATF | Advanced Toroidal Facility |
| ATF | Activating Transcription Factor |
| ATF | Automatic Transmission Fluid |
| ATG | Derivative Thermal Anal. |
| ATG | Antithymocyte Globulin |
| ATH | Aluminum Trihydrate |
| ATIR | Attenuated Total Reflectance Infrared |
| ATL | Adult T-cell Leukemia |
| ATLAS | A Toroidal LHC Apparatus |
| ATLAS | Airborne Tunable Laser Absorption Spectrometer |
| ATLAS | Atmospheric Laboratory For Applications And Science |