| Acronym | Meaning |
| AOAC | Association Of Official Analytical Chemists |
| AOB | Analytical Operations Branch (OSWER) |
| AOB | Adiabatic Oxygen Bomb (calorimeter) |
| AOB | Ammonia Oxidizing Bacteria |
| AOC | Administrative Order Of Consent |
| AOC | Areas Of Concern |
| AOC | Assimilable Organic Carbon |
| AOCI | Airborne Ocean Color Imager |
| AOCS | American Oil Chemists' Society |
| AOD | Argon Oxygen Decarburization |
| AODC | Acridine Orange Direct Count [of Bacteria] |
| AODC | Australian Oceanographic Data Center |
| AODP | Advanced Ocean Drilling Program |
| AOEC | Association Of Occupational And Environmental Clinics |
| AOES | Actinometry Optical Emission Spectroscopy |
| AOGCM | Atmosphere-ocean General Circulation Model |
| AOI | Automatic Optical Inspection |
| AOIPS | Atmospheric And Oceanographic Information Processing System |
| AOM | Angular Overlap Model |
| AOMA | American Occupational Medical Association |
| AOML | Atlantic Oceanographic And Meteorological Laboratory (of NOAA |
| AOP | Advanced Oxidation Processes (hydroxyl Generation) |
| AOQL | Average Outgoing Quality Limit |
| AOS | Alpha Olefin Sulfonate (surfactant) |
| AOSTRA | Alberta Oil Sands Technology And Research Authority |
| AOT | Aerosol OT (surfactant) (reg: 577117) |
| AOT40 | Accumulated Ozone Exposure Over A Threshold Of 40 Ppb |
| AOT60 | Accumulated Ozone Exposure Above A Threshold Value Of 60 Ppb |
| AOTF | Acousto-optic Tunable Filter |
| AOTS | Acoustic Optic Tunable Scanning |
| AOU | Apparent Oxygen Utilization |
| AOVB | Atomic Orbital Valence Bond (molecular Wave Function) |
| AOX | Adsorbable Organohalogens |
| AP | Appearance Potential |
| AP-PCR | Arbitrarily Primed Polymerase Chain Reaction |
| APA | Administrative Procedure Act |
| APA | Air Pathway Analysis |
| APA | Adiabatic Perturbation Approxn. (for Quantum Calcns. On Fermi Superfluids) |
| APA | Alkaline Phosphatase Activity |
| APACHE | Accelerator For Physics And Chemistry Of Heavy Elements |
| APAD | 3-acetylpyridine Adenine Dinucleotidepar |
| APAES | Appearance Potential Auger Electron Spectroscopy |
| APAF | Apoptosis Protease Activating Factor |
| APAR | Absorbed Photosynthetically Active Radiation |
| APB | Antiphase Boundary (crystal Chemistry) |
| APC | Air Pollution Control |
| APC | American Plastics Council |
| APC | Activated Protein C |
| APC | Adenomatous Polyposis Coli |
| APC | Anaphase-promoting Complex |