| Acronym | Meaning |
| ANPR | Advance Notice Of Proposed Rulemaking |
| ANRHRD | Air Noise Radiation Health Research Division |
| ANS | Advanced Neutron Source |
| ANS | American National Standard |
| ANS | American Nuclear Society |
| ANS | Autonomic Nervous System |
| ANSI | American National Standards Institute |
| ANSS | American Nature Study Society |
| ANSTO | Australian Nuclear Science And Technology Organization |
| ANTA | 5-amino-3-nitro-1H-1 |
| ANTARES | Advanced Nuclear Thermal Rocket Engine Simulator |
| ANTHEM | Plasma Simulation Code |
| AO | Arctic Oscillation |
| AO | Announcement Of Oppurtunity |
| AO | Abnormal Occurrence |
| AO | Administrative Order |
| AO | Air Overseas |
| AO | Annotated Outline |
| AOC | Area Of Concern |
| AOC | Award Of Contract |
| AOC | Area Of Contamination |
| AOD | Argon-oxygen Decarbonization |
| AOG | Acid Offgas |
| AOG | Augmented Offgas |
| AOIPS | Atmospheric And Oceanographic Information Processing System |
| AOL | Airborne Oceanographic LIDAR |
| AOM | Annual Operating & Maintenance |
| AOML | Atlantic Ocean Meteorological Laboratory |
| AON | Atmospheric Odd Nitrogen Species Project |
| AOO | Anticipated Operational Occurrence |
| AOP | Abnormal Operating Procedure |
| AOP | Annual Operating Plan |
| AOPR | Associated Offices Of Primary Responsibility |
| AORD | Astronaut Operations Requirements Document |
| AOS | Advanced Orbiting System |
| AOS | Archive And Operations System |
| AOS/VS | Advanced Operating System/virtual Storage |
| AOSO | Advance Orbiting Solar Observatory |
| AOSP | Advanced Onboard Signal Processor |
| AOV | Air-operated Valve |
| AP | Anomalous Propagation |
| AP | Attached Payload |
| AP | Access Permit |
| AP | Action Plan |
| AP | Administrative Procedure |
| AP | Amparo |
| AP | Assigned Personnel |
| APA | Allied Pilot's Association |
| APA | Administrative Procedures Act |
| APA | American Planning Association |