| Acronym | Meaning |
 | 2.5G | Second-and-a-half-generation Mobile Telephone System |
| 2D | Two |
| 2G | Second |
| 3D | Three |
| 3G | Third |
| 3I | Investors In Industry (venture Capitalists) |
| 3M | Minnesota Mining And Manufacturing Company |
| 4D | Four-dimensional (usually Three Spatial Plus One Temporal Dimension |
| 4G | Fourth-generation Mobile Telephone System |
| 4GL | Fourth-generation Programming Language |
| 4WD | Four Wheel Drive (cf. AWD) |
| A | Acro (joke Acronym) |
| A | Ampere |
| A | Atto |
| A&S | Armament & Standardisation |
| A.K.A. | Also Known As |
| A/C | Air Conditioned/Conditioning - Aircraft |
| A/D | Analog/Digital |
| A/S | Anti-submarine |
| A2ATD OR A?ATD | Anti |
| A2C2 OR A?C? | Army Airspace Command And Control |
| A4 | Many |
| AA | Achieved Availability - Alcoholics Anonymous -Armed Forces Americas (postal Symbol) - Aruba (FIPS 10-4 Territory Code) -Assembly Area - Automobile Association- Avenue Of Approach |
| AA | Afar Language (ISO 639-1 Code) |
| AA | Attoampere |
| AAA | Administration |
| AAAI | American Association For Artificial Intelligence |
| AAAS | American Association For The Advancement Of Science |
| AAAV | Advanced Amphibious Assault Vehicle |
| AAC | Army Air Corps (British Army) |
| AAD | Analogue-Analogue-Digital (CD Quality) - Australian Antarctic Division |
| AADAC | Alberta Alcohol And Drug Abuse Commission |
| AADC | Area Air Defence Commander |
| AADD | Atlanta Alliance On Developmental Disabilities |
| AADT | Annual Average Daily Traffic Flow |
| AAFC | U.S. Army Air Corps Ferrying Command (1941-1942) - All-America Football Conference (1946?1949) |
| AAFES | Army And Air Force Exchange Service |
| AAI | Applied Artificial Intelligence |
| AAIS | Advanced Airborne Interceptor Simulator |
| AAM | Air-to-Air Missile |
| AAN | American Academy Of Nursing |
| AANEAS | Anglo-Australian Near-Earth Asteroid Survey |
| AAODL | Atmospheric Aerosols And Optics Data Library |
| AAP | Advanced Acquisition Plan - Allied Administrative Publication - American Academy Of Pediatrics |
| AAPC | Accounting And Auditing Policy Committee |
| AAR | After-Action Review - Association Of American Railroads |
| AAR | Afar Language (ISO 639-2 Code) |
| AARP | (i/s) American Association Of Retired Persons (now Officially Known Only By The Initials) |
| AAS | Advanced Automation System - American Astronomical Society (double-A-S) |
| AASPEM | Advanced Air-to-air System Performance Evaluation Model |
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