- Air separation
- Air suspension
- Air traffic control
- Airbags
- Aircraft
- Aircraft cabin
- Aircraft design process
- Aircraft engine
- Aircraft flight dynamics
- Aircraft pilot
- Airflow
- Airfoils
- Airframes
- Airlift pump
- Airliners
- Airport apron
- Airports
- Airspeed
- Akaganeite
- Alabaster
- Albite
- Alclad
- Alcohol
- Alcohol dehydrogenase
- Alcoholism
- Aldehydes
- Aldol reaction
- Aldoses
- Aldosterone
- Alfalfa
- Algae
- Algal blooms
- Aliasing
- Alite
- Alizarin
- Alkali feldspar granite
- Alkali metals
- Alkali soil
- Alkali-aggregate reaction
- Alkali-carbonate reaction
- Alkali-silica reaction
- Alkaline battery
- Alkaline earth metals
- Alkaline hydrolysis
- Alkalinity
- Alkalis
- Alkaloids
- Alkalosis
- Alkane
- Alkenes
- Alkoxides
- Alkyl groups
- Alkylammonium
- Alkylation
- Alkyne metathesis
- Alkynes
- Allergic
- Allergic contact dermatitis
- Allergic rhinitis
- Allosteric regulation
- Allotropes of carbon
- Allotropes of oxygen
- Allotropes of phosphorus
- Allotropy
- Alloy 20
- Alloy steel
- Alloy wheels
- Alloys
- Alluvial fans
- Alluvium
- Alpha case
- Alpha decay
- Alpha particles
- Alternative fuel
- Alternative fuel vehicles
- Altitude
- Alum
- Aluminate
- Aluminium
- Aluminium alloys
- Aluminium bronze
- Aluminium compounds
- Aluminium foil
- Aluminium gallium arsenide
- Aluminium hydride
- Aluminium hydroxide
- Aluminium nitrate
- Aluminium nitride
- Aluminium oxide
- Aluminium phosphate
- Aluminium phosphide
- Aluminium recycling
- Aluminium silicate
- Aluminium smelting
- Aluminium sulfate
- Aluminium-air battery
- Aluminium-copper alloys
- Aluminium-lithium alloys
- Aluminium-magnesium alloys
- Aluminium-silicon alloys
- Aluminized steel
- Aluminosilicate
- Aluminum can
- Alzheimer's disease
- Amalgam
- Amateur radio
- Ambient pressure
- Amblygonite
- American Iron and Steel Institute
- Americium
- Amides
- Amine gas treating
- Amines
- Amino acids
- Ammonia
- Ammonia production
- Ammonia solution
- Ammonium
- Ammonium carbonate
- Ammonium chloride
- Ammonium fluoride
- Ammonium hexachloroplatinate
- Ammonium nitrate
- Amorphous calcium carbonate
- Amorphous carbon
- Amorphous metal
- Amorphous silicon
- Amorphous solids
- Amount of substance
- Ampere
- Ampere-turns
- Amperometry
- Amphibious vehicles
- Amphoterism
- Amplifier
- Amplitude
- Ampoules
- Amylase
- Anabolism
- Anaerobic corrosion
- Anaerobic oxidation of methane
- Anaerobic respiration
- Analcime
- Analogy
- Analytic geometry
- Analytical chemistry
- Anaphylatoxins
- Anaphylaxis
- Anatase
- Anatomy