China 
Africa 
- Asparagine synthetase
- Aspartame
- Aspartate aminotransferase
- Aspartate transaminase
- Aspartic acid
- Aspartic proteases
- Aspect ratio
- AspectJ
- Asperger syndrome
- Aspergillosis
- Asperities
- Asperity
- Asphalt
- Asphalt concrete
- Asphalt plant
- Asphalt shingles
- Asphaltene
- Asphaltite
- Asphaltum
- Aspheric lens
- Aspherical
- Asphyxiant gases
- Asphyxiants
- Aspiration
- Aspirator
- Aspirin
- Assays
- Assembly
- Assembly language
- Assembly line
- Assertions
- Asset
- Asset allocation
- Asset health management
- Asset integrity management
- Asset management
- Asset management plan
- Asset tracking
- Assignment
- Assignment operator
- Assignment problem
- Assimilation
- Assimilative capacity
- Assist
- Assisted living
- Assistive listening device
- Assistive technology
- Associated Legendre polynomials
- Associated petroleum gas
- Association mapping
- Association rule learning
- Associative
- Associative arrays
- Associative property
- Astatine
- Astaxanthin
- Aster
- Asterisk
- Asthma
- Astigmatism
- Astragalus
- Astrid
- Astringents
- Astrocytes
- Astrocytoma
- Astrogliosis
- Astronauts
- Astronomical clocks
- Astronomical unit
- Astrovirus
- Asymmetric
- Asymmetric carbon
- Asymmetric cell division
- Asymmetric dimethylarginine
- Asymmetric flow field flow fractionation
- Asymmetric induction
- Asymmetric valley
- Asymmetric warfare
- Asymmetrical
- Asymmetry
- Asymptomatic
- Asymptomatic carriers
- Asymptotes
- Asymptotic
- Asymptotic analysis
- Asymptotic distribution
- Asymptotic expansions
- Asymptotic homogenization
- Asymptotic theory
- Asynchronous
- Asynchronous communication
- Asynchronous learning
- Asynchronous serial communication
- Asystole
- Atacamite
- Atan
- Ataxia
- Ataxia telangiectasia
- Atelectasis
- Atherectomy
- Atheroma
- Atherosclerosis
- Athlon 64
- Atkinson cycle
- Atlas
- Atmega328
- Atmega328p
- Atmosphere
- Atmospheric carbon dioxide
- Atmospheric chemistry
- Atmospheric correction
- Atmospheric dispersion modeling
- Atmospheric electricity
- Atmospheric entry
- Atmospheric methane
- Atmospheric optics
- Atmospheric pressure discharge
- Atmospheric pressure photoionization
- Atmospheric science
- Atmospheric temperature
- Atmospheric window
- Atmospheric-pressure plasma
- Atolls
- Atom
- Atom economy
- Atom interferometry
- Atom laser
- Atom optics
- Atom probe
- Atomic
- Atomic absorption spectroscopy
- Atomic age
- Atomic broadcast
- Atomic carbon
- Atomic clocks
- Atomic coherence
- Atomic diffusion
- Atomic emission spectroscopy
- Atomic energy
- Atomic formula
- Atomic mass
- Atomic nucleus
- Atomic number
- Atomic orbitals
- Atomic packing factor
- Atomic radius
- Atomic ratio
- Atomic sentences
- Atomic spacing
- Atomic units