- aggregate
- air resistance
- air clamp
- air hose
- albedo
- air conduction
- air coolant
- air friction
- alkaline battery
- allowance
- alpha counter
- alpha radioactive
- alpha rays
- alpha-active
- alpha-emitter
- alphaspectrometer
- amplidyne
- amplitude
- analyser
- analyses
- anion
- anisotropic
- anisotropic waveguide
- angular momentum
- anisometric
- anisotropy
- anodal
- anode
- anodic
- anomaly
- amphoteric
- analysis
- antifreezing dope
- angular oscillation
- anode coating
- anodic protection
- antimatter
- antiproton
- antiproton atom
- antiresonant
- aplanat
- aplanatic
- aplanatism
- apochromat
- apochromatic
- apochromatism
- apogean
- apparent
- arc breakdown
- astigmatic
- astronomic telescope
- asynchronism
- astatic
- astigmatism
- asynchronous transmission
- atmometer
- atmospheric drag
- atomic binding
- atomic reactor
- arc back
- arc discharge
- arc ignition
- area moment
- attenuate
- attenuation
- attract
- attraction
- attraction force
- autodyne
- automatic carriage
- audion
- avalanche
- axial scanning
- axial shield
- axis
- Babinet compensator
- bable clamp
- back diffusion
- back emf
- back pressure
- background blur
- background vorticity
- backhand welding
- backing paper
- backlash
- backlight
- backlighted photo
- backlit
- backlog
- backscatter
- backscatter effect
- backscatter error
- backscatterer
- auxiliary battery
- axial sensitivity
- axial magnification
- balance
- balance bob
- balance brush
- balanced draught