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- aral sea
- aram
- aramaean
- aramaic language
- aramco
- aran islands
- arapaho
- ararat, mount
- aras river
- araucanian
- araucaria
- arawak
- arawakan languages
- arbitrage
- arbitration
- arbor
- arboretum
- arboriculture
- arborvitae
- arbovirus
- arbutus
- arc de triomphe
- arc furnace
- arcade
- arcadia
- arcadian league
- arch
- archaea
- archaeology
- archaeopteryx
- archbishop
- archean eon
- archer fish
- archery
- archetype
- archimedean screw
- archimedes' principle
- architecture
- archives
- archon
- archosaur
- archytas
- arctic archipelago
- arctic circle
- arctic fox
- arctic ocean
- arctic tern
- ardea
- ardennes
- ardhanarisvara
- ards
- arecibo observatory
- areopagus
- ares
- arginine
- argon
- argonauts
- árgos
- argument from design
- arhat
- århus
- aria
- ariadne
- arianism
- aries
- aril
- arion
- aristides
- aristides the just
- aristocracy
- arithmetic
- arjuna
- ark of the covenant
- arkona
- arkose
- arlington
- arm
- armada, spanish
- armadillo
- armageddon
- armagh
- armagnac
- armed islamic group
- armenian
- armenian language
- armenian massacres
- armistice
- armory show
- armour
- armoured vehicle
- armoury practice
- arms control
- army
- arnhem land
- aromatherapy
- aromatic compound
- aroostook war
- arrastra
- arrest
- arrhythmia