China 
Africa 
- Aromatic amines
- Aromatic amino acids
- Aromatic compounds
- Aromatic hydrocarbons
- Aromaticity
- Aromatization
- Arpanet
- Arrangement
- Array factor
- Array gain
- Array slicing
- Arrays
- Arrhenius equation
- Arrhenius plot
- Arrhythmia
- Arrow
- Arrow of time
- Arrow's Impossibility Theorem
- Arroyos
- Ars operon
- Arsanilic acid
- Arsenate
- Arsenate minerals
- Arsenate reductase
- Arsenic
- Arsenic acid
- Arsenic contamination
- Arsenic contamination of groundwater
- Arsenic minerals
- Arsenic pentoxide
- Arsenic poisoning
- Arsenic trichloride
- Arsenic trioxide
- Arsenic trisulfide
- Arsenical copper
- Arsenite
- Arsenobetaine
- Arsenolite
- Arsenopyrite
- Arsenous acid
- Arsine
- Art gallery problem
- Artelys Knitro
- Artemether
- Artemisinin
- Arterial blood
- Arterial blood gas
- Arterial input function
- Arterial line
- Arterial occlusion
- Arterial spin labeling
- Arterial spin labelling
- Arterial stiffness
- Arteries
- Arteriogenesis
- Arterioles
- Arteriosclerosis
- Arteriovenous fistula
- Arteriovenous malformation
- Arteriovenous oxygen difference
- Artery
- Artesian wells
- Artesunate
- Arthralgia
- Arthritis
- Arthrodesis
- Arthropathy
- Arthroplasty
- Arthroscopic surgery
- Arthroscopy
- Arthrospira platensis
- Articular
- Articular cartilage damage
- Articular cartilage repair
- Articular disc
- Articular processes
- Articulated
- Articulated robots
- Articulated vehicles
- Artifacts
- Artificial
- Artificial bee colony algorithm
- Artificial bone
- Artificial cardiac pacemaker
- Artificial cells
- Artificial cerebrospinal fluid
- Artificial dielectrics
- Artificial enzymes
- Artificial gravity
- Artificial heart
- Artificial heart valve
- Artificial horizons
- Artificial immune systems
- Artificial intelligence in healthcare
- Artificial kidney
- Artificial leather
- Artificial life
- Artificial lift
- Artificial ligaments
- Artificial lung
- Artificial muscles
- Artificial neural networks
- Artificial neurons
- Artificial organs
- Artificial pancreas
- Artificial photosynthesis
- Artificial reefs
- Artificial respiration
- Artificial seawater
- Artificial skin
- Artificial stone
- Artificial sweeteners
- Artificial tears
- Artificial transmission line
- Artificial turf
- Artificial ventilation
- Artificiality
- Artillery
- Artisanal mining
- Artwork
- Aryl
- Aryl groups
- Aryl halides
- Aryl radical
- Arylsulfatase
- Asbestiform
- Asbestos
- Asbestos abatement
- Asbestos cement
- Asbestos-related diseases
- Asbestosis
- Asbolane
- Ascariasis
- Ascend
- Ascending aorta
- Ascending colon
- Ascending node
- Ascites
- Ascorbate peroxidase
- Ascorbic acid
- Ascorbyl palmitate
- Asepsis
- Ash
- Ash ponds
- Ashing
- Ashlar
- Asialoglycoprotein
- Asialoglycoprotein receptor
- Asparaginase
- Asparagine