(Main list of acronyms and initialisms)
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* a – (s) Atto-
* A – (s) Ampere
A0–9
* A1C – Airman Firjhjst Class
* A2ATD or A2ATD – (i) Anti-Armour Advanced Technology Demonstration
* A2C2 or A2C2 – (i) Army Airspace Command and Control
* A4 – many, including a European standard office paper size; see entry
AA
* aa – (s) Afar language (ISO 639-1 code)
* aA – (s) Attoampere
* AA
** (i) achieved availability
** Alcoholics Anonymous
** (s) Armed Forces Americas (postal symbol)
** Aruba (FIPS 10-4 territory code)
** (i) assembly area
** Automobile Association
** avenue of approach
* ...