Definition from Wiktionary, the free dictionary
- Advanced Aircraft Corps
- Aerial Ambulance Company
- Aeronautical Advisory Council
- Agility Association of Canada
- Alameda Arts Council
- Alaskan Air Command
- All-American Canal
- Aluminum Annodizers Council
- Allumnae Advisory Center
- American Academy of Criminalists
- American Airlines Center
- American Alpine Club
- American Anglican Council
- American Archery Council
- American Association of Criminology
- American Atheist Center
- Amphibian Airplanes of Canada
- Anglo-American Corporation
- (military) Anti-aircraft Corps
- (UK, US) Army Air Corps; Army Air Corps
- Army Apprentices College
- (Portugal) Associação Académica de Coimbra, Coimbra Academic Association.
- Association of Alabama Camps
- Association of American Choruses
- Atlanta Athletic Club
- Australian Agricultural Company
- Australian Apprentiship Centre
- Australian Association of Chiropractors
- (historical) Auto Avia Contruzione, Enzo Ferrari's first company.
- Automotive Advertisers Council
- Auxiliary Artillery Corps
- Auxiliary Army Corps
- American Cement Corporation, stock exchange symbol.
AAC (plural AACs)
- Initialism of advanced audio coding.
- Initialism of aerated autoclaved concrete.
- (aviation) Initialism of aeronautical approach chart.
- Initialism of affirming anglican catholicism.
- Initialism of aircraft armament change.
- Initialism of anesthesiologist assistant certified.
- Initialism of Anno ante Christum., in the year before Christ.
- (military) Initialism of anti-aircraft command.
- Initialism of area advisory committee.
- Initialism of augmentative and alternative communication.
Proper noun 
AAC (plural undefined)
- Initialism of American Alumni Council., now known as Council for Advancement and Support of Education
- (dated) Initialism of Association of American Colleges., now known as Association of American Colleges and Universities
- (dated) Australian Air Corps, now known as Royal Australian Air Force
- Initialism of Associação Académica de Coimbra (“Coimbra's Academic Association”).