- apparent absorption
- apparent albedo
- apparent expansion
- apparent permeability
- apsides
- arc
- arc lamp
- arcing contacts
- arcing time
- area
- argon liquefier
- aqua pump
- aqueous
- arbitrariness
- arc extinction
- arc resistance
- arcing shield
- argon
- argon liquefaction
- armature head
- armature winding
- artificial ageing
- artificial ear
- as
- aspect
- assembler
- assessment
- asteroid
- astrobiology
- astrochemistry
- astrographic camera
- astrolabe
- astronomical almanac
- astronomical nutation
- astronomy
- asymptotic behaviour
- atmosphere
- applied mathematics
- approximated
- approximation
- arc chute
- armature
- armature reactance
- arithmetic average
- armature contact
- armature resistance
- armature-control
- arrival
- artificial atmosphere
- artificial climate
- artificial echo
- aspheric surface
- astatic coils
- artificial ground
- artificial snow
- ascension
- aspherial
- astatic pair
- astigmatism
- astral
- astronautics
- astronomical reference
- asynchronous machine
- athermanous
- atmospheric condensation
- atom cluster
- atomic beam
- atomic collision
- atomic number
- atomic paramagnetism
- atomic physics
- atomic structure
- atomic vibration
- atomization
- atomizing chamber
- atomizing nozzle
- arbitrary
- arc current
- arc splitter
- Aroclor
- aromatic hydrocarbon
- arrester
- artificial hypothermia
- artificial line
- artificial load
- artificial mouth
- assembly cooler
- associated field
- association
- astatic system
- asterism
- astigmatic lens
- astrograph
- astronaut
- asymmetrical distortion
- atmosphere composition
- atmospheric air
- atmospheric conditions
- atmospheric moisture
- atomic arrangement