| Acronym | Meaning |
 | AR | Argon |
| ARA | Arabic Language (ISO 639-2 Code) |
| ARAG | Advanced Research And Assessment Group |
| ARB | Administrative Review Board |
| ARBA | Army Review Boards Agency |
| ARC | Arc-second Raster Chart - Association For Retarded Citizens - Appalachian Regional Commission |
| ARCENT | United States Army Central Command |
| ARDA | Advanced Research And Development Activity (became DTO 2006) |
| ARE | UK Admiralty Research Establishment (?1991) -United Arab Emirates (ISO 3166 Trigram) |
| ARF | ASEAN Regional Forum |
| ARG | Aragonese Language (ISO 639-2 Code) |
| ARG | Argentina (ISO 3166 Trigram) |
| ARI | Acute Respiratory Infection - U.S. Army Research Institute For The Behavioral And Social Sciences |
| ARL | UK Admiralty Research Laboratory (1921?1977) - U.S. Army Research Laboratory - UIUC Aviation Research Laboratory |
| ARM | Anti-Radiation Missile -Armenia (ISO 3166 Trigram) |
| ARNET | Acquisition Reform Network |
| ARP | Address Resolution Protocol - Applied Research Programme |
| ARPA | Advanced Research Projects Agency |
| ARRC | ACE Rapid Reaction Corps |
| ARRF | Allied Rapid Reaction Force |
| ARS | Agricultural Research Service -Argentine Peso (ISO 4217 Currency Code) |
| ARSC | Arctic Region Supercomputing Center |
| ARSPACE | U.S. Army Space Command |
| ART | Adaptive Resonance Theory - Anti-Retroviral Therapy |
| ARTCC | Air Route Traffic Control Center |
| ARWG | Army Research Working Group (are-wig) |
| AS | Adult Swim -Assamese Language (ISO 639-1 Code) - Attosecond |
| AS | American Samoa (postal Symbol; ISO 3166 Digram) - Australia (FIPS 10-4 Country Code) |
| AS | Arsenic |
| AS | Attosiemens |
| ASA | AcetylSalicylic Acid - American Standards Association - Army Security Agency - Atlantic Southeast Airlines |
| ASAP | As Soon As Possible |
| ASARS | Advanced Synthetic Aperture Radar System - Airborne Search And Rescue System |
| ASB | Associated Student Body - Alternative Service Book (Church Of England) |
| ASBL | Association Sans But Lucratif (French Non-profit Organisation) |
| ASBO | Anti-Social Behaviour Order |
| ASC+T | Anti Slip Control + Traction |
| ASCAP | American Society Of Composers |
| ASCC | Army Service Component Commander - Automatic Sequence Controlled Calculator - Air Standardization Coordinating Committee |
| ASCIET | All Services Combat Identification Evaluation Team |
| ASCII | American Standard Code For Information Interchange |
| ASD | (U.S.) Assistant Secretary Of Defense - Autism Spectrum Disorder |
| ASD(C3I) | (U.S.) (Command |
| ASD(FMP) | (U.S.) Assistant Secretary Of Defense (Force Management Policy) |
| ASD(HA) | (U.S.) Assistant Secretary Of Defense (Health Affairs) |
| ASD(ISA) | (U.S.) Assistant Secretary Of Defense (International Security Affairs) |
| ASD(ISP) | (U.S.) Assistant Secretary Of Defense (International Security Policy) |
| ASD(LA) | (U.S.) Assistant Secretary Of Defense (Legislative Affairs) |
| ASD(PA) | (U.S.) Assistant Secretary Of Defense (Public Affairs) |
| ASD(RA) | (U.S.) Assistant Secretary Of Defense (Reserve Affairs) |
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