- 1-butanol
- 1-propanol
- 15-crown-5
- 18-electron rule
- 2-butanol
- 2-butoxyethanol
- 2-hexanol
- 2024 aluminium alloy
- 21-crown-7
- 3-methylpentane
- 3-phosphoglyceric acid
- 3D computer graphics
- 3D printing
- 3G
- 4G
- 5052 aluminium alloy
- 5083 aluminium alloy
- 6061 aluminium alloy
- 6063 aluminium alloy
- 7075 aluminium alloy
- A-weighting
- A36 steel
- AASHO Road Test
- ABS
- ABWR
- AC
- AC power
- ACS
- ADAS
- AES
- AFM
- AFM phases
- AHWR
- ALD
- AM
- AM broadcasting
- AMD
- AMS
- AOD
- AP1000
- ASHRAE 55
- AWG
- Abacus
- Ablation
- Abrasion
- Abrasives
- Absolute scale
- Absolute zero
- Absorbed dose
- Absorption
- Absorption cross section
- Absorption refrigerator
- Absorption spectroscopy
- Abutments
- Acanthite
- Accelerated curing
- Acceleration
- Accommodation
- Accuracy and precision
- Acetal
- Acetamide
- Acetate
- Acetic acid
- Acetic acid bacteria
- Acetic anhydride
- Acetone
- Acetylene
- Acetylide
- Acicular
- Acid dissociation constant
- Acid hydrolysis
- Acid rain
- Acid strength
- Acid-base homeostasis
- Acid-base reaction
- Acid-base titration
- Acidity regulator
- Acidosis
- Acne
- Acoustics
- Acoustoelastic effect
- Acre
- Acrylamide
- Acrylate polymer
- Acrylic acid
- Acrylic fiber
- Actinium
- Action potentials
- Activated carbon
- Activation energy
- Active fire protection
- Active laser medium
- Active matter
- Active noise control
- Active site
- Active suspension
- Acute kidney injury
- Acute radiation syndrome
- Acute toxicity
- Adamantane
- Adaptation
- Adaptive cruise control
- Adaptive immune system
- Adaptive optics
- Addition reactions
- Adhesion
- Adhesive
- Adhesive bonding
- Adhesive tape
- Adiabatic process
- Admittance
- Adobe
- Adsorption
- Adulterants
- Advanced composite materials
- Advanced gas-cooled reactor
- Advection
- Aeolian processes
- Aeration
- Aerial refueling
- Aerobatics
- Aerobic organisms
- Aerodromes
- Aerodynamic center
- Aerodynamic force
- Aerodynamics
- Aerogel
- Aeronautics
- Aerosols
- Aerospace
- Aerospace engineering
- Aerostats
- African plate
- Aft phases
- Afterburners
- Agar
- Aggregate
- Aggregate base
- Agitator
- Ailerons
- Air barrier
- Air conditioning
- Air cooling
- Air embolism
- Air entrainment
- Air filters
- Air guns
- Air handler
- Air pollution
- Air pump