| Acronym | Meaning |
| ACSI | Accelerated Strategic Computing Initiative (for Fusion Reactor) |
| ACSL | Advanced Continuous Simulation Language |
| ACSP | Acetyl Cyclohexylsulfonyl Peroxide |
| ACSR | Aluminum Conductor Steel Reinforced |
| ACSTM | Alternating Current Scanning Tunneling Microscopy |
| ACSV | Adsorptive Cathodic Stripping Voltammetry |
| ACT | Alternative Control Technique |
| ACT | Acoustic Charge Transport |
| ACT | Activated Clotting Time |
| ACT | Activated Complex Theory |
| ACTFEL | Alternating Current Thin-film Electroluminescent |
| ACTG | AIDS Clinical Trials Group |
| ACTH | Adrenocorticotrophic Hormone (reg:9003-56-9) |
| ACTI | Advanced Computational Technology Initiative |
| ACTS | Advanced Communications Technologies And Services |
| ACTS | Advisory Committee On Toxic Substances |
| ACTS | Asbestos Contractor Tracking System |
| ACV | Alternating Current Voltammetry |
| ACWA | American Clean Water Association |
| ACWM | Asbestos Containing Waste Material |
| ACZS | Alumina-chrome-zirconia-silica Refractory |
| AD | Alzheimer Disease |
| ADA | N-(2-acetamido) Iminodiacetic Acid |
| ADA | American Diabetes Association |
| ADA | N-(2-acetamido) Iminodiacetic Acid |
| ADA | American Diabetes Association |
| ADA | N-(2-acetamido) Iminodiacetic Acid |
| ADA | American Diabetes Association |
| ADA | N-(2-acetamido) Iminodiacetic Acid |
| ADA | American Diabetes Association |
| ADA | Adenosine Deaminase |
| ADA | Adipic Acid |
| ADAF | Advection-dominated Accretion Flow |
| ADAM | Angular Distribution Auger Microscopy |
| ADAM | Average Depleted Antarctic Meltwater |
| ADAM | A Disintegrin And Metalloprotease Domain |
| ADAP | Angular Distribution Of Annihilating Photons |
| ADAPTS | Air Deliverable Antipollution Transfer System |
| ADARD | Acid Deposition And Atmospheric Research Division |
| ADAS | Acid Deposition Assessment Staff |
| ADAS | Angular-dependent Auger Spectroscopy |
| ADC | Analog-to-digital Converter |
| ADC(N) | Algebraic Diagrammatic Construction (method In Nth-order Perturbation Theory) |
| ADCC | Antibody-dependent Cellular Cytotoxicity |
| ADCI | American Die Casting Institute |
| ADCL | Accredited Dosimetry Calibration Laboratory |
| ADCP | Acoustic Doppler Current Profiler |
| ADDC | Ammonium Diethyldithiocarbamate |
| ADDL | Amyloid Beta-derived Diffusible Ligands |
| ADE | (thermal) Adsorption-desorption Equilibrium |