- Anchor bolts
- Anchor plates
- Anemia
- Anesthesia
- Anesthetic
- Aneurysm
- Aneutronic fusion
- Angiogenesis
- Angiography
- Angiotensin
- Angle of attack
- Angle-resolved photoemission spectroscopy
- Anglesite
- Angstrom
- Angular displacement
- Angular frequency
- Angular momentum
- Anhydrous
- Anisotropy
- Ankerite
- Annealing
- Annulus
- Anode
- Anodic protection
- Anodizing
- Anorexia
- Anosmia
- Anoxic waters
- Antacids
- Antarctica
- Antenna
- Anterior pituitary
- Anthocyanin
- Anthracene
- Anthracite
- Anthraquinone
- Anti-VEGF
- Anti-fog
- Anti-fouling paint
- Anti-graffiti coating
- Anti-inflammatory
- Anti-lock braking system
- Anti-reflective coating
- Antibiotics
- Antibonding molecular orbital
- Anticaking agents
- Anticlines
- Antidepressants
- Antiferromagnetism
- Antifreeze
- Antihistamines
- Antiknock agent
- Antimonide
- Antimony
- Antioxidants
- Antipyretics
- Antiseptics
- Antoine equation
- Anvils
- Aortic
- Apatite
- Aplite
- Apnea
- Apollo program
- Apoptosis
- Apothem
- Apparent magnitude
- Apparent temperature
- Apparent viscosity
- Aqua regia
- Aqueducts
- Aqueous homogeneous reactors
- Aqueous solutions
- Aquifers
- Arachidonic acid
- Aragonite
- Aramid
- Arc lamps
- Arc welding
- Arch dam
- Arch-gravity dam
- Archaea
- Arches
- Archimedes' principle
- Archimedes' screw
- Architects
- Architectural engineering
- Architectural glass
- Argentite
- Argillite
- Argon
- Arithmetic
- Arkose
- Aromatic amines
- Aromatic compounds
- Aromaticity
- Arrhenius equation
- Arrhythmia
- Arsenate
- Arsenic
- Arsenic poisoning
- Arsenopyrite
- Arsine
- Arterioles
- Artery
- Articulated vehicles
- Artificial cardiac pacemaker
- Artificial heart
- Artificial intelligence in healthcare
- Artificial seawater
- Artificial stone
- Artillery
- Aryl groups
- Asbestos
- Ash
- Ash ponds
- Ashlar
- Aspartame
- Aspartic acid
- Aspect ratio
- Asperity
- Asphalt concrete
- Asphalt plant
- Asphaltene
- Asphyxiant gases
- Assembly language
- Assembly line
- Assimilation
- Astatine
- Asthma
- Astrid
- Astringents
- Astronauts
- Asymptotes
- Ataxia
- Atherosclerosis
- Athlon 64
- Atmospheric chemistry
- Atmospheric dispersion modeling
- Atmospheric electricity
- Atmospheric entry
- Atmospheric science
- Atmospheric temperature
- Atom
- Atom probe
- Atomic absorption spectroscopy
- Atomic clocks
- Atomic diffusion
- Atomic mass
- Atomic nucleus