China 
Africa 
- Applied element method
- Apply
- Apprenticeship learning
- Approval voting
- Approximate computing
- Approximate entropy
- Approximate identity
- Approximate inference
- Approximate string matching
- Approximation
- Approximation algorithm
- Approximation error
- Approximation theory
- Approximations of
- Apraxia
- Apriori algorithm
- Aprons
- Aprotic
- Aprotinin
- Apt
- Aptamers
- Apyrase
- Aqua regia
- Aquadag
- Aquaplaning
- Aquaporin
- Aquaporin-1
- Aquaporin-4
- Aquastat
- Aquatic ecosystems
- Aquatic life
- Aquatic species program
- Aquatic toxicology
- Aqueducts
- Aqueous homogeneous reactors
- Aqueous humor
- Aqueous humour
- Aqueous solutions
- Aqueous two-phase systems
- Aquifer storage and recovery
- Aquifer test
- Aquifers
- Aquo complex
- Arabinogalactan
- Arabinogalactan protein
- Arabinoxylan
- Arachidonic acid
- Arachnoid granulations
- Aragonite
- Araldite
- Aramid
- Arbiters
- Arbitrary waveform generator
- Arbitrary-precision arithmetic
- Arbovirus
- Arbuscular mycorrhiza
- Arbutin
- Arc
- Arc blow
- Arc fault
- Arc flash
- Arc lamps
- Arc length
- Arc routing
- Arc welding
- Arch dam
- Arch-gravity dam
- Archaea
- Archaerhodopsin
- Archard equation
- Arches
- Arches of the foot
- ArchiCAD
- ArchiMate
- Archie's Law
- Archimedean spiral
- Archimedes' principle
- Archimedes' screw
- Architects
- Architectural design
- Architectural designer
- Architectural drawings
- Architectural engineering
- Architectural glass
- Architectural patterns
- Architecture
- Architecture framework
- Arcjets
- Arctangent
- Arctic
- Arctic Oscillation
- Arctic amplification
- Arctic climate
- Arcuate nucleus
- Arduino
- Arduino Nano
- Arduino Uno
- Ardupilot
- Area Control Center
- Area density
- Area navigation
- Area of a circle
- Area of a triangle
- Area sources
- Area under the curve
- Areal velocity
- Areca nut
- Arecoline
- Arenite
- Areola
- Arg max
- Argatroban
- Argentite
- Argentometry
- Argillaceous
- Argillic alteration
- Argillite
- Arginase
- Arginine
- Arginine decarboxylase
- Arginine deiminase
- Arginine dihydrolase
- Arginylglycylaspartic acid
- Argon
- Argon laser
- Argonaute
- Argument maps
- Argument of a function
- Argument of latitude
- Argument principle
- Argyria
- Argyrodite
- Arias intensity
- Aridity index
- Aristolochic acid
- Arithmetic
- Arithmetic coding
- Arithmetic functions
- Arithmetic mean
- Arithmetic progression
- Arity
- Arkose
- Arm64
- Armadillo
- Armatures
- Armrest
- Arnold's cat map
- Arnoldi iteration
- Aromatase
- Aromatic