AT
- aT – (s) Attotesla
- At – (s) Astatine
- AT – (i) Anti-Tank – (s) Ashmore and Cartier Islands (FIPS 10-4 territory code) – Austria (ISO 3166 digram) – (i) Advanced Technology (IBM Personal Computer/AT)
- ATA – (i) Advanced Technology Attachment (IBM Personal Computer) – Afghan Transitional Authority – Allen Telescope Array – (s) Antarctica (ISO 3166 trigram)
- ATAC – (a) Air Transportable Acoustic Communications
- ATACMS – (p) Army TACtical Missile System
- ATAPI – (a) Advanced Technology Attachment Packet Interface
- ATAS – (a) Air-To-Air Stinger (missile)
- ATAT – (a) Aiken Tactical Airsoft Team / Star Wars vehicle
- ATB – (i) Active Time Battle – (s) British Antarctic Territory (ISO 3166 trigram; merged with AQ in 1979)
- ATC – (i) Air Traffic Control – Air Training Corps – U.S. Air Transport Command (1942–1948)- Athletic Trainer, Certified (www.nata.org)
- ATCCIS – (i) Army Tactical Command and Control Information System
- ATESS – (i) Aerospace and Telecommunications Engineering Support Squadron
- ATF – (s) French Southern Territories (ISO 3166 trigram) – (i) (U.S.) Bureau of Alcohol, Tobacco, Firearms and Explosives (from its previous name, which did not include "Explosives")
- ATG – (s) Antigua and Barbuda (ISO 3166 and FIFA trigram)
- ATGM – (i) Anti-Tank Guided Missile
- ATGW – (i) Anti-Tank Guided Weapon
- ATI – (i) Allied Telesyn International, Inc. – (a) Array Technologies Incorporated. (ATI Technologies)
- Atisbo – a Local Government Area in Nigeria named for constituent communities (Ago-Are, Tede, Irawo, Sabe, Baasi, Ofiki and Owo)
- ATK – (p) Attack
- ATL – (p) Atlanta, Georgia, the largest city and capital of Georgia
- ATLAS – (a) All-Terrain Lifter Army System – (p) Assessment Tool for LAnd Systems
- ATM – (i) Automated Teller Machine – Asynchronous Transfer Mode – At The Moment
- ATN – (s) Dronning Maud Land (ISO 3166 trigram; merged with AQ in 1983)
- ATO – (i) Air Tasking Order – Australian Taxation Office
- ATP – (i) Adenosine TriPhosphate – Allied Tactical Publication – Ammunition Transfer Point
- ATSD – (p) (U.S.) Assistant to the Secretary of Defense
- ATSDR – (i) Agency for Toxic Substances and Disease Registry
- AT&T – (i) American Telephone and Telegraph (U.S. company, also known as "Ma Bell")
- ATV – (i) All-Terrain Vehicle
- ATWA – (a) Air, Trees, Water, Animals
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