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- association of caribbean states (acs)
- associative law
- assumpsit
- assus
- assyria
- astarte
- aster
- aster family
- asteroid
- asthenosphere
- asthma
- astigmatism
- astrobiology
- astrolabe
- astrology
- astronaut
- astronomical unit (au)
- astronomy
- astrophysics
- asturias
- asvamedha
- aswan high dam
- asylum
- asymmetric synthesis
- asymptote
- at&t inc.
- atacama desert
- atalanta
- atargatis
- ataxia
- atchafalaya bay
- atchison, topeka and santa fe railroad co.
- atelectasis
- athabaskan languages
- atheism
- athena
- atherosclerosis
- athlete's foot
- athletics
- atlanta campaign
- atlanta compromise
- atlanta journal-constitution, the
- atlantic charter
- atlantic intracoastal waterway
- atlantic languages
- atlantic monthly, the
- atlantic ocean
- atlantic salmon
- atlantic, battle of the
- atlantis
- atlas
- atlas mountains
- atlas rocket
- atman
- atmosphere
- atmospheric pressure
- atoll
- atom
- atomic bomb
- atomic number
- atomic physics
- atomic weight
- atomism
- aton
- atonality
- atonement
- atp
- atresia and stenosis
- atreus
- atrial fibrillation
- atrium
- atrophy
- atropine
- attainder
- attention
- attention deficit (hyperactivity) disorder (add or adhd)
- atthakatha
- attic
- attica
- attis
- attitude
- attorney general
- attorney, power of
- aubusson carpet
- auction
- audion
- audit
- auditorium
- audubon society, national
- auger
- augite
- augsburg confession
- augsburg, league of
- augsburg, peace of
- augustan age
- augustinian
- auk
- aulos
- aurgelmir
- aurignacian culture