China 
Africa 
- Antigens
- Antigorite
- Antihistamines
- Antihypertensive
- Antihypertensive drugs
- Antiknock agent
- Antimalarial
- Antimatter
- Antimetabolites
- Antimicrobial
- Antimicrobial chemotherapy
- Antimicrobial peptides
- Antimicrobial resistance
- Antimicrobial surfaces
- Antimonate
- Antimonial
- Antimonide
- Antimony
- Antimony pentafluoride
- Antimony pentoxide
- Antimony telluride
- Antimony trichloride
- Antimony trioxide
- Antimony trisulfide
- Antimutagens
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- Antiozonant
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- Antiplane shear
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- Antipsychotic
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- Antiresonance
- Antiretroviral
- Antisense
- Antiseptics
- Antiserum
- Antistatic agent
- Antisymmetric
- Antisymmetric relation
- Antisymmetric tensor
- Antithetic variates
- Antithrombin
- Antithrombotic
- Antitoxin
- Antiviral drugs
- Antivirals
- Antivirus software
- Antiwear additives
- Antoine equation
- Antrum
- Anuria
- Anvils
- Anxiolytic
- AnyLogic
- Anycast
- Anytime A*
- Anytime algorithm
- Aortic
- Aortic aneurysm
- Aortic dissection
- Aortic regurgitation
- Aortic rupture
- Aortic stenosis
- Aortic valve
- Aortic valve repair
- Aortic valve replacement
- Apache
- Apache Cassandra
- Apache Flink
- Apache Giraph
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- Apache HTTP Server
- Apache Hadoop
- Apache Hive
- Apache II
- Apache Jena
- Apache Kafka
- Apache Lucene
- Apache Mahout
- Apache Pig
- Apache Solr
- Apache Spark
- Apache Storm
- Apache Subversion
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- Apatite
- Aperture
- Aperture synthesis
- Apex
- Aphakia
- Aphanite
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- Aphasia
- Apheresis
- Apical dendrite
- Apical dominance
- Apigenin
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- Aplastic anemia
- Aplite
- Apnea
- Apochromat
- Apocrine
- Apocrine sweat glands
- Apodization
- Apollo
- Apollo program
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- Apoptosis
- Apoptosome
- Aporphine
- Apothem
- App
- Appalachian Basin
- Apparent magnitude
- Apparent oxygen utilisation
- Apparent polar wander
- Apparent source width
- Apparent temperature
- Apparent viscosity
- Apparent weight
- Append-only
- Appendectomy
- Appendicitis
- Appendicular skeleton
- Appendix
- Appetite
- Apple
- Apple CarPlay
- Apple II
- Apple Watch
- AppleScript
- Applicability domain
- Application architecture
- Application framework
- Application security
- Application server
- Application software
- Applications of artificial intelligence
- Applications of nanotechnology
- Applications of virtual reality
- Applied arts